I found riding in the rain a bit easier in NY when the rain was frequent and I'd stay off the raod while it was just starting up, let grease and stuff wash away. Here in the desert when things are wet, more than likely because of sprinklers it is really slick.

other slippery things are sawdust and leaves. Also wood is super duper slick, those old school wooden bridges may look nice but they suck when wet, even a little damp and say goodbye to traction.

Watch out especialy at intersections where slow moving cars drip the most oil.

Don't lock up that front wheel! A little practice in dirt will have you a bit better in the rain, being light on the bike helps, if you are stiff it will be to your detriment.

above all be careful, if you feel fatigued, which can happen sooner in the wet from concentration, take a break under a bridge or in a nice warm coffee or pizza place.

good luck