Getting pregnant is a little like potty training children - you don't control it.
We tried for three years. We did ovulation predictors, artificial insemination, sperm spinning, clomid, robitissun, surgery, diet changes, herbs, relaxation exercises. A chiropractor assured me all it would take is an adjustment or two (I didn't believe her, but what the hell, I was trying to take care of back pain - might as well let her try). Taking Charge of Your Fertility is a good book for understanding your cycles - although she tends to be a little flippant that "everyone" gets pregnant using her method eventually unless there is a real problem - and that can be very frustrating when you are doing "EVERYTHING" and every month you get the same negative result.
After three years we were told "we don't know why you aren't getting pregnant. In cases like yours there is a 50/50 chance you will spontenously get pregnant in the next ten years."
We adopted a baby from Korea and then hit the "lucky" part of the 50/50 chance without trying. (Concieved in January, born in September - 36 weeks 2 days).