Lots of options, all of which say something about the practitioner. More importantly, they’re intended to say something about the practitioner:
Flip-flop. This method is subtle and says, “I don’t care too much about social conventions but still care. Also, I don’t wear flip flops in the shower.”
Boat shoe. “I believe in the finer things in life, to the point that it doesn’t matter to me to use a boat shoe to prop a door open because I have a lot of boat shoes.”
Crate/water bottles. “Please come into my room, I finally found something heavy enough to prop the door other than a chair.”
Door prop purchased from school bookstore. “I like things to work but not terribly well.”
Duct tape string to wall. “I love Rube Goldberg machines, booby traps, and WWII-era advertising: That gosh-darn duct tape can do anything! Heck, it’ll even stop a Kraut’s bullet!” (I’m of German descent and figured kraut is better suited to a joke than the Japanese alternative from the same era. You’re welcome)
Michael Jackson doll. “I’m different. And not just a little different. I really go out on a limb and do my own thing. I sometimes caress my own face with a replica Michael Jackson glove, and there’s a pic of me as a kid dressed up as him. My alarm clock plays “Thriller,” which I think is just hysterical.”
Door closed. Typically it means “I’m at class, sleeping, changing, there’s no one around.” But in some cases, “I missed the memo about door-propping and visiting other rooms/joining clubs and seeking people out, so I will be confused as to why I am known as “Brown T-shirt guy” for a month until finally appearing one day. I will be met with a sense of awe and wonder by people who have seen me in the bathroom two or three times, and girls who thought my presentation in the Freshman Experience class was cute. I’m not shy, I swear. I say “I swear” a lot. Would you like me to go back to my room now?”
I personally used the water bottle method. As visitors/room occupants reduce water bottle quantity over time, more visitors wind up coming to room, meaning door doesn’t have to be propped as wide. Downside: you wind up thirsty. Real thirsty, real quick.
Also recommended: a Bludger, to get those funny kids from 4th floor Ravenclaw to stop by again
Southern Boulevard 