Tips for a girl
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.
So if you have intercourse 2 or 3 days before ovulation, you have more chance of having a girl. Probably only the female sperm cells will survive the waiting time. A less deep penetration during a man's ejaculation increases the chances of having a girl. Sperm cells will be deposited further away from the cervix, which is advantageous for the 'stronger' female sperm cells.
The chances of having a girl will increase if the woman does not have an orgasm. The woman's orgasm makes the environment less acidic, which is advantageous for the male sperm cells. If the environment stays acidic it's to the benefit of the female sperm cells.