Drills are essential to succeeding. There are different ones you can do individually or with a team, but whatever the situation, you must be drilling if you expect to win. It is extremely hard to improve otherwise.
The best drill: Practice rounds
Practice rounds are very helpful. You learn more about your case and other arguments, how they interact with your opponents, what is easy to collapse on, what issues exist. The more practice rounds you can do, the better. You can practice every skill for debate, and get better at debating that topic in the process. You should try to get in at least one round in on each side before major tournaments, but the more rounds you do, the better.
Where can I debate?
Debating people in-person around your skill level or better is your best bet for improvement. When you debate in-person, you're as close as possible to the format of a real debate round, and you can work on presentation skills more effectively. Ask around your school to see if teachers are willing to let you use their room after school. However, that's not always possible, especially in smaller schools.
One place I've debated a lot is online, on the public forum discord server. People discuss arguments and are willing to debate, where you'll hop in a call and have a round over the internet. You can find people with varying skill levels and debate styles, from circuits from every corner of the country.
If there aren't people at your school to debate, you can reach out to other schools in the area and see if they're willing to debate. Message some of the debaters on Facebook, email their coach (if applicable), or ask them at local tournaments. Most are willing to debate to get the practice themselves, as debating their teammates can get a little stale.
Individual Speech Redos
Speech redos are the best drill you can do on your own. Take a flow from a past round, and pick a speech you gave. Think about what you could do better, and what places you made mistakes. Re-give the speech and try to improve it from last time.
Redos can be very complicated or very simple. Here are ten different types of redos you can give, adopted from a lecture on redos from Jeff Miller at NDF.
1. Better version
This is the simplest type, just redo the speech. The important thing you have to do is define what "better" means in the context of this speech. Are you trying to cover up a mistake? Stutter less? Group more efficiently? An easy way to get this set up is to take notes during the RFD, writing down the places you could've improved. Then, when starting your redo, consciously think about and/or write down the areas where you want to improve before you start prepping it. Writing down your goals for the redo help focus you in your preparation for the drill.
2. Different circumstances
Most debates aren't decided in the final focus, which is usually a speech that's a matter of "did they finish the job," conclude based on the events earlier on in the debate. As a result, sometimes when giving redos, you couldn't improve as much without having made different decisions earlier on in the round. In this redo, you'll take a speech and re-give it based on those differing circumstances. Maybe you assume you didn't drop a turn, maybe you assume your partner made different weighing. Give the speech as if everything was going well. It'll help you when you make a stupid mistake, but know there's places your redo could improve the speech.
3. Different strategy
Go for a different argument! Make a different weighing mechanism! Extend different defense! This redo helps you with your in-round strategic decisions. What would've happened if you collapsed differently? Would it make the summary easier or harder? This is also helpful for getting to know the way it feels to go for different arguments, getting practice using a different strategy than you normally do. Finally, this redo is helpful in rounds where you've debated teams who are worse than you, since you can see if you made the best decision, even if you won the round in the end.
4. Extreme time constraints
Create pressure for your speech! For example, maybe you had a rebuttal that wasn't very efficient. Give the same rebuttal, with the same arguments and the same analysis, in less time. Gradually work your way down to three, two minutes, to fix the inefficiencies and make more concise wording. This also forces you to think of your best arguments.
5. Opposition on steroids
Think carefully about the round, and what your opponents could've done to make your job a lot harder. Redo your speech if your opponents were way better than they were in the round. When you crush another team, you don't really improve. Maybe they just drop something obvious and the debate ends by you just extending it in summary and final focus. Assume your opponents are much better than they are, and make a redo where the round was much tougher to win.
6. Experimental techniques
Think about a technique-based skill, especially applicable to second-half speeches. Maybe you have a habit of using a voter format, or straight line-by-line, so try the other or try both at the same time. See if a different approach would address issues with the speech or make some arguments easier. Don't just try to copy the best teams, but develop your own style of debate. You should be comparing yourself to a version of yourself from some time ago.
7. Extemporaneous redo
Take a speech from a while ago. Look at the flow, and try to remember what went wrong. Based on the knowledge you had from the other speech, just re-give the speech to the best of your ability. This exposes what you actually know, rather than just you memorizing some nice line of rhetoric for the debate. It'll also help show how similar arguments are across the different topics, with the wording changed up a bit, making you more adaptable.
8. Switch sides
Give the other team's speech against you! See what your opponents could do to win the debate against you. What strategies can they make to make sure you would've lost? This makes you take the other perspective on arguments, and helps test your own arguments as well. See what your opponents could've done to smack you.
9. Switch speaker positions
This is self-explanatory. This can help when you feel partnership tensions, and when you're both too afraid to admit and confront the tension, it might help to give each other's speeches. Can you re-give the summary to do it better, can you give a better rebuttal? This will help the second speaker see how the argument is extended in summary, to adapt their final focus to be more parallel. It's also helpful for the first speaker to give the final focus to see how the semantics differ when trying to close the debate down, which will help them set it up better in summary.
10. Transcription redo
Record your speeches, and listen back to them. Transcribe the speech you gave, every speech tic and filler word, and read through what you said exactly. You'll notice you said a lot of sentences that are incoherent and make zero sense. It helps when you have rounds where everything made perfect sense in your head, but the judge had no idea what you were talking about. Revise the speech like a paper, then re-give it. This helps identify a lot of problems. This can also help with crossfires, if you want to practice that as well.
For clarity and speaking ability, I can't recommend the pen drill enough. The idea behind this drill is that if you speak under conditions that are more difficult than a normal speech, when speaking in-round you'll be more effective. This drill involves you putting a pen horizontally in your mouth, so that it sticks out from each side of your mouth. This puts a blockage for your tongue - it'll have to go above or below the pen in order to speak. The pen drill makes you better at enunciating your speeches, so just practice saying whatever with the pen in your mouth. This is something you can do in the shower.
Drilling with PF Videos
PF Videos is a youtube channel with many, many recordings of rounds from tournaments by good debaters. Watching rounds is a good way to see other peoples' debate styles, but you can do a lot of different drills with the videos that are available.
Everyone can always improve on their flowing. Watch a round and flow it, very simple. This is most effective when watching rounds that aren't in the same style as tournaments you attend or rounds you debate. If you debate a lot of lay rounds with big-picture issues, then watching rounds with very quick, flow-oriented debaters would help you improve, and vice versa. You can also see if you'd come to the same decision as the judges, and evaluate why you would or wouldn't.
After watching and flowing rounds, you can give redos for the people debating! You can choose to pause the round before a particular speech, deliver it as you would if you were in the round yourself, then watch and see what they did differently. Alternatively, you can watch them deliver the speech, think about what they did wrong or things you could improve on, and redo the speech. In some recordings, the RFD is included, so you have an even bigger advantage. Watch the RFD, take notes on different improvements that could be made to the speeches to inform your redo.
PF Videos has a lot of very old videos, ones from topics you have almost certainly not debated. This gives you a good opportunity to practice making analytical responses, as you wouldn't have done any research on the topic beforehand. Watch the constructives and first crossfire, and then give a rebuttal to the opposing case using only weighing of the other case and analytical responses. Not knowing the topic or the case you're going against is not an excuse for a poor rebuttal. The same applies to a first speaking looking to frontline responses on a topic they've never heard about. Practicing in this manner helps you when facing arguments you might not have heard of before on topics you'll debate at real tournaments.
Specialized practice rounds
Practice rounds alone are great for practicing everything at once, but sometimes you want to practice certain skills. Here are some twists you can put on the classic practice round to help you learn a bit more.
These are very simple, impromptu debates on a random topic. While you can make them more serious, I find you can improve skills the most when debating wackier or even abstract topics. Resolutions about pop culture or topics like "should Chas quit playing Call of Duty" can help simplify the complex arguments made in a debate round, especially for newer debaters. In the abstract, weighing-focused debates like "what's better, sandwiches or fire alarms?" can organically bring out important arguments to be applied to real rounds. These debates flow better with half speech times, e.g. 2 minute constructive/rebuttal instead of 4. An alternative speech time format I've found works well with very simple topics is Aff 0:30, Neg 0:45, Aff 0:25, Neg 0:35, Aff 0:15.
Pick a topic that people don't have a lot of opinions on already, and let people only use the topic's Wikipedia article for evidence. This should help you with making more analytical responses, and analytical offensive arguments, since you have such a small base to build your case on. This is more applicable to debate formats with half speech times.
Have a round that's normal, but has big gaps between the speeches. You have plenty of prep to get ready for the perfect responses, the best arguments you can make against your opponents for each speech. (To stop people from making tons of blippy responses, I recommend imposing a limit on the number of arguments you can make in each speech, e.g. 7 or 8 distinct arguments in rebuttal.) The big gaps can help make the best strategic decisions, think about what the biggest issues with your opponent's arguments are, etc.
Stop and judge
Have a practice round, but after every crossfire and at the end of the round itself, write down an RFD for each team. What justification would the judge give to vote for either of them? At the end of the round, compare the different RFDs given at different times. What decisions were made in different speeches to change the RFDs? Make your arguments consistent. Assuming you and your partner have coordinated your strategy well, the RFD for your team should be similar each time.