Great game, loved creating my own character on there!
I think there's really only three controls - pass, shoot, boost. Hold down the boost button to move faster and pull off better tricks. To do the best slam dunks, your play should be "on fire" - that is you need to score three baskets in a row without reply from the other team with the same player. The game will shout "He's on fire!!!" or something similar, and you can pull off some incredibly audacious stuff once your guy is on fire!
The game is a bit slow by default, so you should put in some cheat codes before each game starts... enter 552 in the "Tonight's Matchup Screen".