I ride back and forth between LA and Sacramento two to three times a month, and rain is part of the game.
Gear up before it starts to rain. Make sure your boots are water proof or get some covers.
Experiment early with your windscreen position and learn at what elevation it will toss most of the rain over your head. For my RT that's about halfway up. More vertical and it spills over into my lap,and any lower and it just goes into my face shield.
Do something about fogging in your visor. Use Cat Crap, spit, pinlock, FogCity, etc. For me, fogging has been the biggest problem. I'm dry as a bone from head to toe, except I have had to crack my visor to clear fogging and I wind up with mist flying in my face.
Rain will build up a bit on your visor; turning your head left, then right, will usually clear the droplets. Some folks use RainX; I don't.
The roads in California have seen plenty of rain as of now, so the greasy, grimy gopher-gut crap is washed away. Still, stay clear of the center of the lane. If you can, ride in the fast lane, as it is cleaner. Be especially wary of overpasses, as the GGG crap doesn't get washed away under the overpass. There is always a transition at the overpass (asphalt to concrete or concrete to concrete, as the under structure of the overpass is heavy concrete) and I've gotten sideways at 70 mph going through an overpass. Admittedly, it was during a first rain of the season.
During the week the drivers on I5 and 101 are pretty much pros and are, for the most part, sane. However, the kids may be going back to school, and they are nuts. Look out for cars filled with young people and boxes. Many of them like to drive at the speed limit, but they do it in the fast lane. Most traffic will want to run at 75-80. The kids doing 70 in the fast lane forces others to pass in the right. If you're running in the slow lane, keep an eye on your six. I actually had a crazy person pass me on my right, in my lane, at about 80 while I was doing 70. Lane sharing is legal, but that was nuts.
If you will be going over the Grapevine, recognize that, even now, there is a potential for snow or freezing rain along about Gorman to Tejon Ranch. Also, the down hill section from Tejon Ranch to Grapevine is very rough. Rough enough that you'll think you've got a flat.
Traction is good on wet roads that are clean - reportedly about 80% of what is available on dry roads. If you can get your head around that, it reduces the pucker factor in corners.
Get glove liners or insulated gloves. It's often cold in the rain. Waterproof gloves often are not! Take two pair of gloves. Tuck the gauntlet part of the glove up the sleeve of your rain gear, lest the rain run down your gear and into your glove, filling your water proof glove with water. No, you may not ask how I know that...
Enjoy, I absolutely love riding in the rain. About three weeks ago I was at the north end of Stockton on I5, in full gear and with my windscreen set just right (it had been a bit misty with spots of light rain). It was about 8:30 and quite dark. I couldn't figure out why the light raindrops wouldn't clear from my visor when I turned my head left and right. A car passed me and, in it's headlights, I could see that I was in a torrential downpour. I was dry and warm and didn't even know it was raining hard.
What route are you taking?
Tom