Motivation had great insight. My trick is park my car, leave it running with the heater on full blast. Suit up, get back in the car for 5-10 minutes until I'm toasty. Then go and dive and try to get in the water as soon as I can. In the summer it really isn't an issue. The water is colder but the sun is out and its nice and warm so you get hot fast on the surface.
If you are cold before you get in the water you aren't going to get any warmer. After a certain amount of time you will get cold in the water and its time to quit the dive and get on the surface. Do not continue diving if you are cold, do not be afraid to call the dive because you are cold, but your buddy still has 1500psi.
For thermal protection, drysuit is the best (which I generally use) After that go for a semi dry, I think they make 9/7/5 or something along those lines that are really toasty. For the price though you might as well get a cheap or used drysuit.
I have a two piece farmer john 7mm that keeps me warm. I got it from a shop getting rid of rental suits for $20, some minor repair and it works well enough to keep me warm when I can't use my drysuit.
Also I don't think you will ever stay "toasty warm" in wetsuit in california, but with the proper gear you can stay fairly warm. I really think your best bet is to pick up a cheaper drysuit. I got a CF200 that had never been used and was a few years old for $650 which is right in the ball park of a really good semi dry. Granted there are a few additional costs associated with it, but if you are paying that much you might as well get the thing that will work the best.