As with most dorm pranks, this endeavor requires some preparation time in order to achieve the best results. When the dorm's occupants are away (and the door is left unlocked) recruit several friends to assist you in blowing up balloons. Ideally you should be able to do this overnight. Blow up enough balloons to completely fill the space of the room, preferably sticking balloons against the wall and furniture too for added annoyance.
This classic prank has been the nightmare of plenty of unsuspecting victims and remains popular throughout colleges. When the subject is away, move every piece of furniture out of her room and hide it somewhere else. Or for added comic effect, leave only a single object in the center of the room -- for example: a pencil. Although this prank can be great for irritating the subject, expect to assist her in returning the room to normal afterwards.
This simple prank works best when you only have a limited amount of time. Go into the subject's dorm and turn every single poster and photograph upside down. If you consistently do this every time the subject is away, it's likely he'll eventually stop trying to correct the posters and just leave them the wrong way round. For additional effect, turn all his furniture upside down too.
Pranks involving the subject's laptop are common in dorm rooms, as possibilities for jokes are almost unlimited. A popular favorite is to take a screenshot of the subject's computer desktop (this is done by hitting the "Prt Scr" key), hiding all their folders somewhere on the computer, and setting the screenshot as their computer background. This will cause confusion and irritation when they're unable to select anything on their desktop.