Get the Timing Right
Other medical experts also believe that Y sperm (boys) swim faster than X sperm (girls). However, girl sperm typically are more robust and live longer in the reproductive tract than the boy sperm.
So what does this theory mean to you? In a nutshell, if you're hoping for a boy you should try to abstain from sex for several days before ovulation and then have intercourse as close to ovulation as possible. This increases the likelihood that the boy sperm (the faster swimmers) will beat out the girl sperm to the egg, and not have to survive too long while waiting around for the egg to show up. You'll have to do some very careful calendar watching or take an ovulation test to determine precisely when you're ovulating.