Growing plants need nitrogen. But this is not the case for blossoming and fruit development. So if you want a few 'early' tomatoes, make sure you discontinue any nitrogen fertilizer. You may still want to fertilize with soft rock phosphate or green sand.
Early fruit ripening can be forced by stress. Thus you may want to deprive a limited number of your plants from water. You may even consider digging them up and replanting them in another location. All this stress will cause the plant to attempt to produce seed ASAP, which will result in an earlier harvest.
We recommend this procedure only on a limited number of plants, as this method will reduce the total production of any plant treated to this regimen.