As many people know (or have heard), the sperm that carry male chromosomes (which will make a boy) can swim faster because they carry less genetic material and are lighter. So though male and female sperm have equal muscular power in their flippers, the lighter ones will travel faster (just like a smaller cyclist will go faster than a larger one, if all other things are equal, because there's less mass to move).
But even if the male sperm outrace the female sperm, the male sperm will die off if the egg isn't in proper position for fertilization, leaving the female sperm clear for landing. Although male sperm are faster, female sperm are like tortoises; they keep pushing ahead slowly, hanging out, and waiting a few days. To have a boy, having sex around ovulation is critical, or the female sperm will win.
Aside from timing it right (and praying), you can do little to influence gender during conception. Some couples may try copulating upside down or in other acrobatic positions because they've heard rumors that you can influence the gender of a child, but the process is more complex than that.