(how) is it possible to choose the biological sex of a child?
The main technique is sperm separation or sorting, where you separate sperm by gender, and only use the gender that you want. Existing techniques aren't perfect. AFAIK, they are typically less than 90% accurate, but it is still better than 50%. Typically, this involves some type of cell sorting device often with some kind of fluorescent tag marker.

The second technique is embryo screening. Often, with IVF, you screen and can confirm the gender that you want before implanting the embryo in the mother.

Links:

http://www.fertility-docs.com/programs-and-services/gender-selection/select-the-gender-of-your-baby-using-pgd.php

http://www.in-gender.com/Gender-Selection/MicroSort/