I know there are books out there on this~
i have a friend who had 3 boys and wanted a girl
it goes by certian months they followed it
and whether or not it works I don't know but she did have a girl!!!!!
The book had things for both to conceive.
That was a while ago but iam sure there are still books out there on this.

I read on the internet that dog style is the best position to have a boy because the man can penetrate fully. If you want a boy have sex about a day before obvulation and all the days after that. This will increase your chances of a boy. Douche with water and baking soda instead of water and vinegar which will make you more acidic inside and decrease your chances of a boy, because the male sperm die in a more acidic atmosphere. The male sperm is faster but die faster also, the female sperm can live a long time but they swim much slower which is why it said you should lye down after intercourse to give the male sperm a chance to reach the egg first. If you stand or sit up it will move the sperm down and a girl is more likely to be born because the male sperm would not reach the egg before dying.