There is a theory out there about male & female sperm (that is, sperm which will create baby boys and sperm which will create baby girls). It seems that male sperm are smaller, weaker, and faster. So with plenty of charting and adherence to the rules, you can influence your babies gender. (Please note: this will not work for a woman with an irregular cycle because there's no way to tell when ovulation happens. Also, if you are having trouble conceiving, trying to influence gender is just narrowing your chances altogether.)
Step one: charting. Primarily, you'll be charting the woman's basil temperature. Every single day. For at least three months. If you're up to it, use ovulation tests around when you think you might be ovulating to pinpoint the date. These two tools together can help a woman tell her most fertile times - the entire basis behind natural family planning.
Step two: timing. For a girl, have intercourse before ovulation. This way, the male sperm will have reached an empty goal and expired while the female sperm are still hanging around waiting for the prize. Start eight days before ovulation; stop about three days before ovulation. For a boy, wait for ovulation. Refrain from intercourse for about four days before ovulation. Have intercourse once on the day of ovulation. Return to refraining for a few days.
Step three: a few extras. Want a girl? Have the guy take a hot bath right before intercourse. Want a boy? The guy should drink a cup of coffee right before intercourse. For a girl, skip the female orgasm (makes the female body more hostile - and the male sperm being weaker, they suffer more). For a boy, attempt maximum penetration to give the male sperm a better head start. Also, try prayer and intention. Intend to have a girl. Pray to have a girl. Some higher power may be listening and grant your desire (of course, sometimes higher powers feel they know better than you what you need - apparently, that's part of the definition of higher power).
Step four: acceptance. Since you're narrowing your window of choice conception days, this can make it more difficult to get pregnant. Even if you follow the rules and recommendations closely, there is no guarantee that you will conceive or that the gender will be affected. This is less a guarantee than a padding of the odds (6 to 4 on a girl anyone?). However your attempt goes, you need to accept what you receive - a baby is going to be adorable, regardless of gender.