The only foolproof method would be PGD (preconception genetic diagnosis) which is a form of in vitro fertilization where the embryo is tested (usually for genetic problems, though in this case for gender) before being reimplanted in the woman’s womb. For the purpose of gender selection this is not usually halachically permitted, absent other (usually medical) factors. (For further discussion on this topic, see “Sex Selection and Halakhic Ethics: A Contemporary Discussion” in the Tradition journal.)

There are other methods touted as increasing the chances of conceiving a specific gender, such as the Shettles method. While it is halachically permissible to try this method, there is little evidence for its efficacy. More important, you should be prepared for the possibility of having another boy, even if you employ the Shettles method.

It is permissible to daven for a specific gender prior to conception and up until 40 days into the pregnancy. We are not aware of any other methods that will increase your chances of conceiving a girl. This is clearly in Hashem’s hands, and we can only accept and appreciate the gifts that He gives us.

B’Hatzlacha!