Time on the water is #1 for sure... nothing competes with experience.
BUT -- Time BEFORE the water is highly important, especially if you are fishing new water. So my #2 way to improve fishing would be research. Putting in due diligence, before fishing new water especially, is key in my opinion. Approach your day on the water with a solid plan based on research of the lake (internet, old magazines, tackle shops, friends, and map study) and appropriate for the seasonal pattern, so that when you get there you're not trying to figure out where to start. Being able to get good start based on the map, and where the fish "should" be is a great step towards a great day. Practicing that when it doesn't matter will help when it does.
I also do the whole "yard fishing", and it's a good way to improve certain skills. I've also done the "only take one type of lure" fishing, but I think that kind of get's tricky. If the conditions and bite are right for lure x, then you will have better success then if they aren't regardless of technique, etc. If you want to work on your topwater skills, but it's a bluebird day, no wind, high sun, clear water... chances are your success will be worse than trying early A.M., on a foggy morning, with a litte wind out of the west.
I would suggest practicing how to effectively fish structure vs how to fish a lure. Take a dock in 12 feet of water, for example. You can learn so much from that first doc you come to IF you can fish it effectively. You can spend a lot of time trying different techniques to throughly examine that structure, and even if you don't catch fish, you will be learning "how" to approach it for the best coverage.
Where as if you are just trying to learn lure "X" you are giving up a lot of opportunity to find fish quite often. Replace doc with offshore hump, or secondary point, or inside creek channel, etc. etc. Yeah, I know that not every (insert place) will be full of fish, but I think you can learn more technique by learning how to throughly vett structure then you learn by only throwing lure "X".
So I guess my practice suggestions are research and to spend time practicing specific structure.