Tips for a boy
The baby's sex is determined during fertilization. The man's sperm cells have either an X or a Y chromosome. The woman's egg-cells have one X chromosome. The sperm cell that fertilizes the egg-cell determines the sex. If a sperm cell with the Y chromosome fertilizes the egg-cell, you get a boy. If a sperm cell with the X chromosome fertilizes the egg-cell, you get a girl.
Female (X) sperm cells live longer, but swim slower. Male (Y) sperm cells swim fast, but live for a shorter time.
If you have intercourse right before ovulation, you have more chance of having a boy. Deep penetration by the man during ejaculation increases the chance of having a boy. The environment nearer to the cervix is less acidic, and therefore it's advantageous for the 'weaker' male sperm cells.
The chances of having a boy increases if the woman has an orgasm first and the man has an orgasm soon after. The woman's orgasm makes the environment less acidic, which is advantageous for the male sperm cells.