In southern California, you should still have plenty of time...
and warmth for the tomatoes to grow, once the flowers stay put. I don't know much about flowers dropping off, but others above, here, have given good advice.
I did read, though, that once the tomatoes have started to go from flowers to tiny green tomatoes, it's helpful to "root prune" them to direct the plant into giving more of the energy to the fruit rather than the roots. You just dig a shovel into the soil about a foot away from the plant, as if you were going to dig it up, but don't loosen the soil. That severs the root so it says to itself "Gee, maybe I should put more energy into my fruit."
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From Cornell's Ag-Experiment Station: "(Cold weather) will also be a good time to root prune the plants. Root pruning means taking a spade and pressing it into the soil about 6-8 inches. Do this in a circle, one foot from the base of the plant. By doing this you will sever parts of the root which will usually hasten ripening."
At least that's what I've read. 'Seems as though it should work, and I plan on doing this in about a week, since the days are getting shorter and the nights are getting cooler.