I heard that to have a boy, some couple only eat vegetables, no oil, no meat; plus living as in a poor conditions, under high pressure, and worry about future --- similar as in a war, more boys coming.
It's working for one couple I knew them in China. However, you need to think about it is very a pity to do it
If you want have a girl, please see followed link, it may help:
(Sorry, I could not find it -- I post it on CFC before. I'll give you the link after I find it.)