Regular watering is key. Sounds like you're being consistent, so that's good.
They like soil to be kept cool, loose and moist. Good container soil is often good for staying loose, but containers are hard to keep moist and cool. You may consider placing the containers into larger one (i.e.- place a plastic one into a larger clay one, or larger ones, depending on how hot and exposed to the sun they are.
Tomato plants turn yellow for a variety of reasons:
1) Not getting enough sun- happens as the plant matures. Not a big deal.
2) Not getting enough nitrogen- usually affects newish growth. If you have a plant with a lot of turgid, healthy and deep green growth, this is probably not your problem.
3) Pests- examine closely for signs of pests (i.e. holes in the leaves).
4) Water- addressed above. I've found that most of the problems with my tomatoes occur when I don't get enough water on them. Generally, this will show up by causing the plant to be spindly, lighter green...can often be confused with nitrogen problems.