Don't Choke The Bar To Death
When holding the bar overhead in the snatch or jerk, it's common for newer lifters to over-grip it as if it's trying to fly away. Your grip overhead should only be as tight as necessary to control the bar and the position of the hand and wrist - no tighter.
A tight grip causes the elbow flexors to tighten, which limits the extension of the elbows (both the speed of extension when punching the bar into position, and the ultimate degree of extension in the final position). It will also prevent the hand and wrist from settling into the proper position.
If you're not sure what your hand and wrist should look like overhead, take a look at the cradle of a squat rack - the tall part is your fingers, the short part is your thumbs, the surface the bar sits in is your palm, and the post is your forearm - your goal is to hold the bar just like that squat rack. I like the way Ursula Garza explains it as well - hold the bar overhead without wrapping your thumb and fingers around it to get it stable, and then grip it in that position.