Install a bunch of browsers. Install Mozilla, Firefox, K-Meleon, Opera8, Opera7.54, MyIE2 (*shudder*), Avant, Netscape 7, Netscape 8, etc. As long as each has it's own program folders with their own temp/cache folders, you should be able to login as a unique visitor in each one.
If you have traditional programming experience, you could probably write a simple executible that cycles through login names, logging names in and out every minute or two. Activity stays in your records generally for 5-15 minutes, so this would allow you to accomplish what you want using only one browser, and you could stay legitimately logged in using another browser.
Still, I don't like the idea of doing this...but if you must, this will do the trick, given you have the right programming resources available