If you wish to have a girl, you should give advantage to the X sperm and make it more demanding for the Y sperm
. This means that positions where shallow penetration is achieved are essential. The difference is that with shallow penetration the sperm will be farther away from the cervix, which means extra traveling and extra odds for Y sperm to die off before they arrive at the destination, as X sperm will manage the distance well enough to conceive a baby daughter.