They eat yeast - from rotting fruit, whatever. Alcohol as well. You can make traps - take an open mouthed jar, create a paper funnel if you don't have a real one, put some cheap beer or wine in the jar. They'll be attracted to the alcohol and won't be able to get out.
saultgirl-I've heard that also. Make sure you run hot water in all of the drains, and espeically concentrate on ones that aren't used too often. And if you have a garbage disposal take the rubber part out of the neck, scrub it with a soapy brush on all sides, and clean the garbage disposal out also by scrubbing it thoroughly, rinse well and put the rubber piece back.
PS buy the cheapest beer and wine. They actually prefer it. When we set up large population cages for experiments, we make up a yeast paste of yeast and the stuff that comes in a box, and they love it.