I agree with Bret Contreras. The best way to fix a crappy movement is to do the movement correctly. This can be accomplished by concentrating really hard and getting feedback - ideally from a coach but at least from video. If the movement to be improved is a resisted one, proper weight selection is key. While merely getting stronger often does not fix movement inefficiencies, inefficient execution certainly prevents strength to be developed in those areas where the inefficiencies take place. This in turn can lead to serious problems in performing an exercise with proper form AND maximal weights. Fixing stuff like this can take a lot of patience and hard work. So better do stuff correctly from the beginning.
Regarding why bad form develops in the first place, there is something to be said about proper and improper exercise selection. Some people convince themselves that one exercise is what they want to "specialize" in or what is "most important" for their goals. Often this is the bench-press, around here it certainly is the squat for many people. But no matter how great that exercise is, it will NOT cover all the bases. And when you are new to training this can be very hard to realize because it's a short-term vs. long-term thing. For example, if a beginner would want to improve his VJ during the next three months, the best thing he could probably do is squat heavy 2-3 times per week and invest the rest of his resources into jump training. Doing other stuff as well might cut into resources that he does not have. But when you plan to train and improve over a long period of time, then doing a well-rounded training program that gets the whole body stronger, is the best thing you can do. Bad motor stuff due to specific weak areas is less likely to develop and you open up a lot of training options for later. Excess strength, that you do not necessarily need in your sport, will rarely hurt you much and it might come in really handy somewhere down the road.
