I tried the ovulation calendar method like some who mentioned here and it really worked for me. For example, if you know for certain that you're going to ovulate on a SATURDAY, begin trying that week before Saturday...up until Wednesday only. It takes a few days for the girl chromosome to make it to the egg when Saturday finally arrives. And by then, the boy chromosome would have died off.
So for example, try to conceive on the Sun., Mon, Tues., and Wed...BEFORE your ovulation is due. NO sex on Thurs., Fri., and Sat., including the week after Saturday). If you're still not pregnant, wait until the next ovulation date. Having sex in the morning is best (well, I hear). It also helps if you have a regular menstruation (not always late, or etc.), that way you're almost always certain when you're going to ovulate.
My first was a girl which was what we wanted...and that was my schedule. For a boy, we tried up until the day of ovulation. And our second is a boy.