If you want to attach a file to send to your chat partner, instead of toggling the attach button, simply drag and drop the file(s) to the text box and hit send!
Rather simple but I hadn't seen the hint.
[Editor's note: True, a simple hint, but still, it deserves a spot in the database...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030113063520659