In a book I've read if you want a certain gender, it's about timing... for a boy you need to bed as close to ovulation as possible, and do not bed the days coming up to ovulation. The female sperm live longer then male sperm, so if you bed the day before and the day of ovulation the chances of conceiving a boy will go up slightly. If you bed 3-4 day prior to ovulation the chances of having a girl will be higher.
This is not garenteed though you can still have girl if you bed the day of etc.. x