Are GMs bad players? How do you step back when someone else is running the show?
Let's answer this one succinctly: yes, GMs are bad players. You know how "too many cooks spoil the broth"? Too many GMs spoil a game. Just as you might come to the game table with one vision of what your game will look like, every other GM comes with that same idea and attitude. And, while this isn't unique to just GMs--everyone has personal preferences, of course!--the act of GMing represents a step towards actualizing that reality. Turning over your stake in a virtual reality is hard!
I do find, however, that my own tendencies as a player help keep the action moving. You see, I'm an instigator. I'm the guy who's always going off on his own, exploring places where he shouldn't be poking around, While I have run into the occasion where new GMs aren't always prepared to deal with my off-the-cuff adventuring, most GMs find that I provide them with ample opportunities to exercise their evil ways, which pleases them to no end.
I also try to make myself available as a rules resource, particularly if we're playing a system that I have a degree of system mastery with. While I do my best to not interfere with the actual act of running a game, if there's a question of rules, I offer to look up page references or cite my own experience in the system. That way, the GM can focus on the in-game situations as they stand, while I can take care of the minutia.
Really? The best thing you can do? Just sit back and enjoy--trust your GM and trust the journey they're going to take you on. Hang up your jaded jacket at the door and dive on in!