My two cents are:
Its not all genetic. There is a strong environmental factor. Just like stressing can cause intersexing (hermes), certian condtions help to nudge a plant into being female. These techniques are to be used during seedling and early veg stages:
Ethylene gas (C2H4). This can be obtained or derived from fruits that are decomposing. The best fruits are ones with soft skins and tha turn brown very quickly when exposed to air such as apples, bananas, and peaches. You can put a banana peel or a sliced apple in with your seedlings to expose them to the gas. The gas is flameble at around 13% air concentration, but this wont be a concern at all. Youd be lucky to have a 1% concentration.
Cool temperatures for the first month. In nature, males are expendable. One male can pollenate and entire field. Cooler temperatures than what the plant expects makes it think that its going to die soon and therefore a female would be the best hope to keep the cycle of life going. The cool temperatures help to influence this survival mechanism that the plant has evolved.
High Nitrogen Diet. I dont know what the pharmacodynamics are but Jorge Cervantes says it helps.
These are not sure fire methods for success. You can give your kid all the books in the world, but it doesnt mean that he will like to read. It could help, but its not a sure thing.
I have also heard things like a birth control pill or menopause HRT medication mixed in with the water can make plants female, but i dont like the sound of all that.
Licorice root contains compounds that inhibit the production of testostorone in humans when consumed. Perhaps a tea made from this could help a plant be more feminine? Since its really a crap shoot, plan on growing 2x as many plants and expect to kill 1/2 of them.