You need some time and male friends for this one, but I've seen this one exectuted just recently and it was amazing.

All you do is replace their clothes, furniture, posters, etc with similar items borrowed from your male friends. Bras are out of course, you just take those, but you replace their undies with whatever you guy friends wear. Same with clothes. ALL clothes...shirts, pants, socks, shoes, everything.

The poster/decoration thing can go etiher way. If you leave up anything they have, there will be a little lag time before they realize that some shit has been done. If you switch it, you'll get to watch the mayhem as they think they're in the wrong room and try to process what the deal is once they realize they're not.

In the version I witnessed, the decorations were switched and the girls who were victims actually went down to the CA crying and pissed because they thought they had been moved out of their room and it had been given to someone else.