Environmental factors are supposedly able to influence the female-to-male ratio... but from what I've heard, you'd need to be growing a LOT of plants in order to see the effects. I think lights with a cooler spectrum (like 'cool' CFLs and MH) used during veg are supposed to lead to more females, as well as lower ambient temperatures during veg... but this may be totally wrong, and again, even if it isn't, you probably wouldn't see any noticeable difference unless you were growing thousands of plants.