"How to Choose the Sex of Your Baby" by Landrum Shettles, MD, PhD
Call me crazy if you will, but we followed this to the T for both of our children and got boys both times. Exactly what we were trying for. My OB laughed at me the first time, but the second time, she was intrigued.
Ovulation cycles, timing of intercourse, position, even diet to a limited extent, have been shown to INCREASE the odds of conceiving one sex or the other. But not guarantee, certainly. And everything in this book has been scientifically researched. The book itself is written by a fertility researcher.
Anyway, short of selective fertilization, this seemed to work well for us.
One last thing. We are Christian and don't think for a minute that we believe we could have changed anything that God didn't want. :-) Ultimately, we prayed for healthy babies both times, but also believed that God helps those that help themselves. ;-) We wound up with two terrific, healthy boys and thank God for them both!