Probably the best solution is to withhold water, that will allow sugars to concentrate in the fruit.
Some growers in CA do that and some do not. You can sure tell the difference in the sweetness of the fruit.

Drought stressed plants produce more flowers and if pollinated, more fruit (viable seed), that has enough stored energy (sugars & starches) to produce and sustain it's offspring.

Thanks ladywindsurfer.

I have read some use a product of crushed granite sprayed on the leaves a few days before harvest and or a solution of kelp to help raise sweetness sightly. The county extension agent thought kelp might help, but there wasn't anything proven except to not water beforehand.