It’s All Right to Be Shallow
Another popular school of thought says that certain sexual positions can increase your odds of having a girl. According to Dr. Landrum Shettles, MD, author of How to Choose the Sex of Your Baby, shallow penetration during intercourse seems to favor girls.
Shettles explains that a woman's vagina is more acidic closer to its entrance, and—as mentioned before—some people believe male-carrying sperm don't fare well in acidic environments. To achieve this more "shallow" penetration, Shettles suggests the missionary position for couples who want to conceive a girl.
Of course, if this gets boring, you can get creative, but keep that shallow principle in mind.