At the start of a season you get 10 new drills. Before beginning your new drill, subtract on point from you 3 point stat, which gives you around a 1000 skill points to spend on anything else. Then, complete a shooting drill by attempting only 3s to get your 3 point attribute maxed out again. This is an easy way to get extra skill points. You can do this anytime you get a drill.
At the Main Menu, scroll down and select "Features", then "Extras, then "Codes". At the codes screen, unlock the following dunks by entering the correspinding codes. Note: After entering the code, load up the Sprite Slam Dunk Showdown and check the Dunk Help menu to view how to perform the new dunks: