First of all I have tried to explain this before. In 2k9 you have to say in front of the offensive person. The KEY on positioning is to watch the offensive persons head movement or shoulders. You can not let the offensive player get a clean look at the basket.
If you watch the offensive player they make a lot of head movements. If they are facing one direction but you are facing another direction then this is considered an uncontested jumper. You do not have to play lock down defense but it helps you to keep your head lined up with the offensive players head. (this is basically squaring up with the offensive player) Now other things come into play like; ratings, size, distance.
Do not expect to lock down superstars or quick players all the time. They make a lot of subtle head fakes to get that clear line of sight at the basket.
You will know when you are playing properly because you will NOT have to jump at jumpers and they will take bad shot, have shot clock violations, be forced away from the paint, commit more unforced turn overs and etc.
Before anyone come in and slaps this down I play on default or superstar with no slider adjustments. I can keep the cpu under 48%, most point guards under 6 assist per game and points in the paint range from mid 20's to low 30's. I play on 7 minute quarters and this style of play works on the online cheesers that use Kobe, James, TMac etc abusers with a double team from time to time to keep them from getting hot.