What is said above, and also, each player at a certain position have a number of abilities that are highlighted and some that are grey. The highlighted abilities train -- much -- faster than the grey abilities. So a tip is to try to select drills that mostly include areas that are highlighted for the player you are training. For example, look at a DC. No attacking abilities are highlighted, if you choose an attacking drill for a DC it will not be very effective for him.
With that said, I still try to get a decent mix. But the emphasize should be on highlighted abilities...