I recently observed my first match at my local range (I was there to use the range but there were matches going on everywhere, so I pulled up a chair).
I was quite impressed with the ability of the vast majority of the competitors and thought that if I was going to join in, I would need some work.
I have started with the the NRA Marksman Qualification Program to give me a standard of progression that I can judge myself with and I have already identified more weaknesses than I thought were available (can anyone say "offhand shooting")...
Having said that, I started looking for drills I could do that would show immediate results before fine tuning form and function.
The drill that has had the biggest impact so far is one I can do almost anywhere: From any position (holster, low ready, etc) close my eyes and bring the firearm to shooting position. Open eyes and see how close I am to target - can I even see my red dot?
That developing muscle memory is, all by itself, a HUGE benefit, especially when shooting timed groups.
Can anyone recommend other drills with big payoffs?