i've played every night since thursday, with the exception of sunday. yeah, claim, i know. we flipped my buddy's cheap pool table over since the frame is not fastened to the table top. felt side down on the legs, and voila, boarded beer pong table. now we don't have to run around chasing stray balls. we will just play a couple leisurely games after we all finish our work. we don't even really get drunk, sometimes its just fun to play without crowds and loud party music.
but to answer your question, we just fill the sink with soap water before we finish for the night, throw cups in sink immediately after playing, rinse after a few minutes and let them dry for the next night. as long as they aren't left out with beer and are cleaned right away they will work forever. sounds cheap, but hey, we're poor. when we need cups for parties, we will do costco.