Taking Charge of Your Fertility (easy read) and The Art of Natural Family Planning (religious perspective, stricter rules) are the two best resources. You just wipe before you use the bathroom every day, which becomes second nature, and take your temperature every morning around the same time after 3 hours of consecutive sleep. The way it works is sperm can't survive more than a few hours if there is no cervical fluid. Cervical fluid is also your indication that your body is preparing for ovulation. So thats how you identify the beginning of your fertile window. After ovulation your temperature will rise. After it has stayed up 4 days (along with a drying up of your cervical fluid), you can be assured the egg is dead and gone and you are infertile for the rest of your cycle. Thats just the general idea of it... the books have all the rules and specifics. I used it the only time I had sex
Definitely plan on using it whenever I start up again ;-) I chart now just for gyn health awareness and to predict my period.