I've impersonated my friends on MSN on several occasions. Usually they're in the middle of a conversation, then leave to go to the bathroom or something. There are two things help me imitate people better:
1) I need to be able to see how they write. Since they're normally in the middle of a conversation, I can just look at everything they've been saying and mimic their grammatic form. Most of the time, though, I know how they write; they usually talk to me on MSN quite a bit.
2) It's really convincing if you refer back to what each of the people said before you took over. If you just get on and start chatting with random people you've got nothing to refer back to; if they've already got the window up, you can at least refer back to earlier points in the same conversation. You could just try starting a conversation from scratch, but it's simply more convincing to know what's been said before.

...and I've never been caught =D