I think the only thing you can do is keep it growing fast and healthy with lots of sun and plenty of potassium rich food. There are chemicals that will induce fruiting in some plants but I'm not sure if it will work on bananas. These same chemicals are said to stop production of male flowers and help start the formation of female flowers on certain hermaphrodite plants. Don't ask me how I know this!
I think we may have talked about it here a while back.
