You should pick the Players you want to train, based on how you want to train. If you plan on doing more 2 position training, to help the base of your team get stronger, you'll want to get players with potentials between, Starter and Allstar. For 1 position training, you'll want an Allstar to Superstar potential, maybe a 21-23 MVP, whom hasn't been trained much, you can get them for cheap, too. Age is the most important of all factors. 18 to 20 year olds train the fastest, then slow in the early 20s, before slowing even more in the mid 20s. After 26, though, for most accounts training takes to long to make it worth while for us. Not to say, you won't experience gains, just not a rate that will help us succeed.
Another successful key is to manage your enthusiasm. This is how much effort your players put into playing defense, in a given game. There are 3 types, Normal, Crunch Time, and Take It Easy. Normal will play at your default enthusiasm, figuring that you've never changed it, it will be a base of 5. Using Crunch Time, you'll will lose 1/3 of your enthusiasm for the next game, but it will increase your enthusiasm 133%. So if you have a enthusiasm of 6, and Crunch time, it will give you an enthusiasm of 8 for the game, but the following game, you'll have an enthusiasm of 4. Playing Take It Easy, you'll only play at 66% of your base enthusiasm. So if you have an enthusiasm of 6, and Take it Easy, you'll play at a 4 enthusiasm. However this will add to your 2, to your 6 enthusiasm for your next game and make it about 8. By looking at your upcoming opponents you can decide, who you can save enthusiasm against, who you must use it on, or who you think you have to play normal against. This is, maybe, the best way to win games, you would maybe ordinarily lose.