The W.H.O. provides this
recipe, which at best if done accurately, gives better
than 80% chance of gender choice.
Know Exactly When You Ovulate: To conceive the sex of your choice, you'll need to conceive either just before ovulation (for a girl) or during - i.e on the day of ovulation (for a boy). This is because X (girl) sperm are much more hardy than their male counterparts and can survive for much longer. Male sperm swim faster but die quicker on the way to “the goal” There are many ways to chart ovulation, but there is very little room for error, so it's important you chose an extremely reliable method. An accurate basal thermometer works best to pinpoint ovulation date.
Correct Positioning: Intercourse positioning is probably the most widely discussed gender selection variable, but it is only one small piece of the puzzle. For optimal natural gender determination, for a girl you need to deposit sperm further away from the cervix (shallow penetration) and closer (deeper penetration) for a boy during moment of ejaculation. This is the easiest variable to control, but it alone is not enough because you could still have a hostile PH and / or be conceiving on the wrong day. To get the results you want, all three variables need to be firmly in place upon ejaculation, but this is easily achieved with a bit of planning and practice.
(The old wives tale - that men
with longer penises tended to have more boys may have substance
i.e. ability to deposit faster swimming male sperm closer to the egg = male sperm wins = boy).
Info on optimizing healthy active sperm production.