Take Your Position
Another popular school of thought says that certain sexual positions can improve your odds of having a boy. In ancient times, beliefs abounded about the positions that would lead to male offspring, such as having intercourse with your heads pointing north or having the would-be mother sleep on the left. Discoveries in science over the centuries have led to somewhat more complex theories about positioning (and a lot else). For one thing, we now know for sure it's the male who determines the sex of a child. The mother always provides an X chromosome in the egg; the father contributes either an X chromosome (yielding XX = girl) or a Y chromosome (yielding XY = boy). Each individual spermatozoa carries either X or Y and the baby's sex depends on which one penetrates the egg most effectively first.
According to Dr. Landrum Shettles, MD, author of How to Choose the Sex of Your Baby, deeper penetration during intercourse seems to favor boys.
He explains that a woman's vagina is most acidic near the entrance, and—as mentioned before—some evidence suggests male sperm don't fare well in acidic environments. Shettles says deeper penetration puts sperm closer to the egg, reducing the distance the sperm have to travel through the acidic vagina. This gives faster-swimming Y-chromosome (male) sperm a head start over the X-chromosome (female) sperm. Therefore, he recommends rear-entry intercourse for couples who want a boy. (Do I really have to clarify "rear-entry"? OK, fine … he's talking about "doggy-style," people. There, I wrote it.)