A: The “Natural” methods of trying to have a baby of one sex or another are outlined in a book by Landrun Shettles, How to choose the sex of your baby, by Doubleday publishers, New York. They rely on the fact that Y sperm are very slightly smaller than X sperm, and possibly faster but more frail.
Scientific methods for separating X from Y sperm prior to artificial insemination exist (the Ericsson or Microsport methods) but the success rates have never been fully proven. The best article is in a scientific journal called Human Reproduction, volume 13, number 9, page 2367 by Fugger and colleagues.
There are, of course, considerable ethical concerns about choosing the sex of a child and many articles are written about this point too. In the UK the practice is rather frowned upon.
The fact that you have had two boys previously is irrelevant as this is probably by pure chance.
Steve Killick, Babyworld Fertility Expert.