Use your right analog stick, push it in the downward direction, and prevent players from getting to the basket, use double teams, rotate, play some zones, mix up your Defensive looks, etc
i agree with Bighort. for me the best way to defend the CPU is to control one of your big men (C's or PF's) depends on who are you comfortable with speed, defense ability, then use the defensive crouch. As a tip, i try to form some sort of a triangular formation with the defender on the ball and the goal. Try pressing the right analog stick down with the turbo button together so even in the defensive stance position (crouch) your controlled big man moves quicker. It takes a lot of practice but once i figured it out, the CPU almost most of the time tries to drive but gets cut off by my controlled big man, passes the ball around and end up shooting outside. Which brings to the next more important part, to get that crucial defensive rebound, (if not you just spoiled one good defensive stand for the damn tip in by the CPU) when the shot is in the air, go for a good position just at the semi circle line and not under the goal, hit shift so you won't be controlling your big man, then once the ball hits the goal, hit shift then jump. I find this very effective and was able to lessen the Offensive rebounds by the CPU.
Just remember, defense is important but just the other half of winning the game. You should have good offensive sets as well and good shot selections to counter the CPU's momentum.