If you can monitor your cycle for several months to find out when you ovulate, you should make love on ovulation day and that should give you the best chance of conceiving a boy. You shouldn't have intercourse for 5 days or so beforehand to allow the sperm count to build up and to ensure there are no girl sperm (which live longer than boy sperm) remaining from the last intercourse.
The theory behind this is that boy sperm are faster and will win the race to the egg (on ovulation day) but girl sperm are slower, have more stamina and are able to hang around longer and lie in wait for the egg.
It's worked for me!