Easy, easy, easy....but you'll need some help. If she works at an office, either her own office or a cube, get a co-worker to cut an orange in half and chuck them behind the lowest drawer of her filing cabinet.
Between the stench, and the eventual fruit flies (and there are _always_ fruit flies, don't ask me how in an office environment), combined with nobody ever figuring out where the odor comes from, it's can't miss.
You could always do the same to her dresser, but since you live there too...
The best part of this trick is that it will take a while for it to kick in, so any expectation of retribution is forgotten in her mind, which makes the smell that much harder to pin on anyone.
-Scott
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