Fertilize with a little nitrogen and some epsom salts. What have you given them and what do you have available?
Tomatoes like P and K, and consume a bit less N in comparison. Look at Espoma brand Tomato-tone fertilizer. It's NPK is 3-4-6. In fact a little bag of that would do you wonders. They do well with Garden-tone too.
All that said I screwed up some of mine by over applying N. I have great big leaves and no yellowing, but flowering has suffered. Oops and oh well.
Also, tomatoes can be finicky about water. Try to maintain a constant moisture level. Not overly wet, not dead pan dry. If they go through wet/dry cycles the fruits can have problems. Dark, dying spots on the bottoms (not end rot) and splitting are two big ones. So keep them watered evenly, and USE MULCH!!!!