If your roommate does something that bothers you, tell them right away: "What you did there was not cool."
Don't save up grievances over the past couple of months and write them in a letter.
Major agreement here. There's nothing worse than being treated to a laundry list of complaints that has been saved up over MONTHS. Especially when some of the complaints were blown so out of proportion because the saver has been stewing.
I would go so far as to say that the laundry list of complaint is something that shouldn't ever be used.
It's basically a really passive aggressive way to unload all your complaints and irritations on someone else, while ensuring that they are not there to defend themselves or respond to accusations.
Passive-aggressive listmakers are just asking for a public fisking.
Oh, here's a rule from my brief roomie career: If you know your roommate's parents are coming for a visit, please only appear before them fully clothed. (My dorm room had only one room, in which we both slept, and she slept in her underwear. My parents came for a visit--of which she was apprised long before--and during their entire visit, she continued to lie in bed in her bra and undies. We kept roughly the same hours, so there was no reason for this behavior.)
Edited to clarify who needed fisking, and also to add my rule input.