Not sure what sort of assessment you're going to, but I'd say don't try to be perfect, just be yourself.
If it's anything like the assessments I've been through, they want to see the potential to learn/be taught. So lets say they get you to do something in the simulator, and it doesn't go well. If they then give you a few pointers and you show a decent improvement, this shows the ability to learn from your mistakes/advice from others.
If the first try is perfect, there is no way to demonstrate this, so I wouldn't go crazy with practice or anything, they're not expecting the finished product for a cadet program, just an ability to learn and be taught.