As a GM, I try different things for different characters.
For instance, standing and moving around the table.
Clenching hands.
Looking about anxiously.
Taking sips of water every few seconds between whispers.
Funny accents and other things usually arent' my way but for some NPC's, I'll talk lower, deeper or faster.
I find that with some character, it's best to Blue Book it. This is where you're basically passing notes around for different aspects. One cool feature of this is your ability to describe a situation different ways. For example, you could have someone come into the room that a few people in the party hate, describe it one way on the paper and pass it to those people, and describe it another way to those that don't know the individual.
For players, I prefer it when the players get at least a little into character. "I say this." is lame. Ever listen to a conversation when the people use terms "like, seems, apparently, could be, it was like." I want to slap those people. Spit it out. "I say that this is the time to go!" vs. "This is the time to go." Describing character movements is acceptable of course. I wouldn't want anyone jumping on my table screaming for their sword.
