Storm Drain Gastrodon (with a Rindo Berry, preferrably) is immune to Water- and Electric-type attacks. Cloud Nine Golduck and Lickilicky can offer some resistance if they have a good partner. Crobat can Sunny Day on the first turn or after Politoed switches in if you predict the switch right while being immune to Fake Out. And--though this is kind of gimmicky--Shedinja frustrates the hell out of the common rain staples Politoed, Ludicolo, and Kingdra. Trick Room, as you said, is also very effective, since from what I've seen you would have to carry an anti-TR Pokemon with little other purpose on the team in order to *effectively* stop its setup consistently. Those have stopped my rain teams anyway, both a Drizzle and nonDrizzle one. If VGC2012 becomes "open," rain-abusers will adapt to these threats probably, but anti-weather teams will have more solutions too as time passes.