From Firefly's post in the top forum:
You might noticed a 'Clear Session' link on the main page, right next to 'Log out' and 'Mark all forums read'.
This is rather simple, yet I like it. If you're just visiting the forum to see if there are new posts, or checking your PM, and don't want your name to appear in Who's Online for 15 minutes after that, just click 'Clear Session', and your name will miraculously disappear!
(I know you can set to be invisible, but this is just for the short term)