Just take it a bit easier as others have suggested. The rain eventually washes the oil off the road, but there's slime from leaves, mud film, and other treacherous stuff even surfaces that you'd expect to be grippy.

On a related note, one of the problems with rain is that it lubricates debris (broken glass, wires, etc) so that it will puncture your tires more easily. Changing wet tires is not as fun because it's easy to introduce debris, so get decent tires. I'm a huge fan of the Schwalbe Marathon Plus. Be aware that durable tires don't grip on wet surfaces as well as good racing tires. Armadillos, a popular commuting tire that is often recommend on BF, has the worst wet weather traction of any tire I've ridden.

Consider not riding your tires at max psi if that's what you normally do and don't corner hard -- it's easy to wipe out.