Frequency and Timing of Intercourse:

Shettles says in order to increase the chances of having a girl, you should have sex everyday from the end of your period up to 2 1/2 to 3 days before ovulation. After this point, do not have unprotected sex until several days past ovulation.

Intercourse further away from ovulation favors the larger slower moving X (girl) sperm. The weaker Y (boy) sperm will die more quickly in the more acidic preovulatory vaginal / cervical secretions and by the time of ovulation there will be a much larger concentration of X (girl) sperm available to fertilize the egg.

Sexual Position:

Shettles suggests that if you are trying to conceive a girl, shallow penetration from your partner, preferably with the missionary position, will deposit the sperm closer to the entrance to the vagina. This area is more acidic than closer to the cervix and acidity will work against the weaker "boy" sperm leaving more "girl" sperm available to fertilize your egg.

Sex and Orgasms:

When trying to conceive a girl, Shettles recommends that you don’t orgasm during sex as the body produces substances after orgasm that makes the vaginal environment more alkaline, which favors the "boy" sperm.

The contractions which accompany an orgasm help move the sperm up and into the cervix, giving the "boy" sperm an extra chance at being available when your egg is available for fertilization.