It was already said here, but I would repeat: over controlling. Number one. I must confess - did that, almost cost me my trainer.
Two: yanking elevator on a take-off, thus stalling the plane... Bad!!!
Three: SIMULATOR IS NOT ENOUGH. Plus - no matter how much time you spent on a simulator - it is different on the field. I guess "reality chaeck" kicks in. Just getting this got feeling of flying (and potantially crashing) real plane is not the same as beeing on a simulator and an ability to "fix" your plane just by clicking couple of buttons.

Aside from that... Good luck and have lots of fun!!!!!