Hey, the In Character Nazi may be more right than you think! ;P Mostly because I know my games tend go on for soooo long that if people stay out of character too much then nothing ever gets done. -_-; I have a few players who like to get to bed before 4 am.
Well, as a player I generally stay in character when I deem it most dramatic. For most actions I just say what I'm doing. As a DM I have to use every trick I can. I have so many NPCs that if I don't find some way to make them seem different then it just looks like I'm talking to myself a lot. And answering.
I do use different voices as a DM, and a lot of it has been unconciously. My voice just automatically deepens or raises for some characters. There was the vampire who spoke in a valley-girl accent (I run a modern game), and most of the players were mortified that voice could come out of me.
My players usually note that they're speaking purely out of character by placing their fists on their heads. This we got from LARPing. ^_^;