Quoting tylersmom + #2:" Research suggests that girl sperm live longer but don't swim as fast. Boy sperm swim fast but don't live ... [snip!] ... its the shettles method that's says if u want a boy have sex closer to ovulation. I have no idea how accurate this is though."
That's what I've read too. If you have sex a couple days BEFORE ovulation you're more likely to have a girl because the girl spermies can last longer, and the boy spermies are fast so they'll get there before you actually ovulate but won't live long enough to still be there when you do ovulate. Also something about the foods you eat, one of them thrives in a more acidic environment or something.