Are you charting your cycle? Get the book "Taking Charge of Your Fertility." It's all about timing your intercourse. Male sperm swim faster and die quicker. If you have sex with your partner on the day you ovulate, the theory is that the male sperm will get to the egg faster than the female sperm. Conversely, if you want a girl, you make love with your partner before you ovulate so that by the time the egg comes floating down the fallopian tube, the male sperm have all raced for the finish, become exhausted, found no egg, and died.
It is not guaranteed, of course, but it's worth trying. If you want a girl, just have sex every single day. It's not only more fun, it works! (take that with a grain of salt). I recommend the book mentioned above VERY highly.