I make sure there's enclosed boxes that help trap body heat, like old tissue boxes, shoe boxes, etc that are mostly closed on all sides. I also always tear strips of newspaper every time I clean their cage and they love that for bedding. I stuff one box and a hammock full of paper shreds and they move it around as they see fit. I was given this hammock a while ago:
http://www.drsfostersmith.com/product/prod_display.cfm?c=6067+28219+17460&pcatid=17460 and my rats LOVE it. There is always at least one ratty curled up, and often times all the rats are in there. I image it stays nice and warm. I also put in strips of fleece and newspaper for extra warmth.
What kind of cage do you have? If you want a semi-permanent heater for your guys, you can use a reptile heatpad that's meant as a permanent fixture in rep cages. it cannot go inside the rat cage, and do not stick it right to the bottom of the cage! I have a small one (I think the smallest or second to the smallest) that I have stuck to a very large ceramic tile. I used it when I have very ill or old rats, and plugged it in for my sick girl who was pts on Sunday. But I think I will keep it on since it's getting a little chillier and I like to keep the window open at a night a bit (I live in CA too, finally some cold weather!) I have a DCN cage, so I put the ceramic tile, heatpad side down facing the floor, on top of the bottom unit's metal grate which is good because it cannot be enclosed, so it has ample air circulation and won't overheat and become dangerous. I sandwich the tile between the wire floor, and the plastic pan, with the tile side on on the plastic side, and the heatpad side on the wire side. It cannot be reached by the ratties at all that way and I feel that it is very safe. It doesn't get too warm, when I stick my hand in the box and touch the fleece floor, it just feels like a rat had just gotten up from a nap, just nicely warm, but not hot at all. I put a shoebox with no lid upside down over the warm spot so if the rats do feel a little chill, they can sleep there. So far no one seems to care much, but when the time comes that I no longer need to use the air conditioner during the day, they may like it.