I think that a good way for a Usual DM Playing as Player to avoid causing trouble is to spell out what he's doing by the rules a bit more than anyone else.
Awhile back, I played (I usually DM), and I could tell that the player was occasionally less than sure about whether people were doing illegal things or not. He knew magic like nobody's business, since he usually played a sorcerer, but he wasn't up to speed on all the combat rules. I was playing a cleric/barbarian, a dwarf who dual-wielded a pair of warhammers. Dang, he was fun...
Anyway, I didn't want to get into rules debates with him, and he and I both knew that I knew the rules really well, so I'd just say, "Hargrf lets out a massive yell and flies across the field of battle, and when he reaches his opponent, his incredible speed and prowess lets him swing his frosty hammers not once but TWICE with amazing fury and dubious accuracy! (Okay, flying, move and attack, second attack from haste, power attack 5)."
Showing a DM that you know what you're doing by the rules makes it less likely that you'll get into stupid arguments. And if he tells me that something I want to do doesn't work, I deal with it. After all, I tell HIM that all the time when I DM...
-tacky