Back Change and One handed Slam
press Left, Forward, during your run up then press X. press L or R during the jump at about the freethrow line.
Back Jump and Clutch Dunk:
Quickly press Up, Left, Down during your run up, then press X. press Down, Up while in the air.
Dunk contest moves:
Select the "New Season" option at the main menu, then press L1, R1, L1, R1, Circle, Square.
Enter the settings screen and select "Rosters". Create a player and copy Kobe Bryant. Change his statistics to 6'9", 225 lbs., and rename him Kenyon Martin. This player can make any reasonable shoot.