There are a few "natural gender selection techniques", which we have written about a few times on SteadyHealth. By far the most popular of these is the so-called Shettles method, which holds that sperm move at different speeds depending on the chromosome they carry. Unfortunately for those who have been trying to get pregnant with a boy, studies have shown no scientific evidence that the Shettles method to conceive a baby of the parents' chosen gender actually works.
Then there is MicroSort, a medical gender selection method that uses DNA screening to find out which sperm will contribute to boys, and which to making a girl. The sperm are then sorted and used in IVF or IUI treatment. While this method is effective, it is not freely accessible to couples who want a boy for non-medical reasons in most countries.
Preimplantatation genetic screening is another method combined with IVF. This time, the testing is carried out after embryos are already created. After finding out which embryos created through IVF are female, and which are male, the female embryos can then be discarded and you can secure your male heir. The poor man's version of this technique would involve using relatively new techniques to find out your naturally conceived baby's gender in the very early stages of pregnancy.
You may find it shocking to realize that, in some cultures, the technique is gaining popularity to carry out a post-conception gender selection. It's possible repulsive perhaps, certainly when this is done purely for cultural and not medical reasons, possible indeed to abort the fetus if it had bad luck, and did not end up a boy. This is, perhaps, a whole different perspective on "selective reduction". The rest of us can make peace with the fact that couples who are trying to conceive have about a 50/50 chance of conceiving a bot or a girl.