Are you talking about the jerks? After you stand up with the bar you can do a very quick eigth squat or something to pop the bar up in the air so you can regrip it. Or better yet when you stand up with it in the first place you can do it quickly enough to throw the bar up a couple of inches so you can regrip it.
Lowering it can be really hard, at least for me. You might just have to lower it and then reset your grip.