Philipp Lenssen posted some tips for Google Talk. Be sure to read the comments, too, for additional tips discovered by readers.
My favorite tip is adding emphasis in messages using the *bold* and _italic_ convention that has been commonly used in plain-text e-mail and Usenet posts for years. I’ve never been one to use a lot of formatting in IM’s, so this is perfect for me. Simple, clean, gets the job done.