There are a few methods of natural family planning.
Rhythm method - This is useful if your periods are regular and of the same length. It basically assumes that you ovulate about 14 days before your period and requires that you avoid sex then.
Ovulation method - This is based on the fact that when ovulating the cervical mucus is like an egg white (clear, slippery, and very stretchy). You should abstain from sex around then.
Standard Day Method - Again useful for women with regular periods. It assumes a 28 to 32 day cycle and requires abstinence on days 8 - 19.
Sympto Thermal Method - Track your basal body temperature every day and once you see a sustained raise for 3 days it means you have ovulated. It is then safe to have sex for the remainder of your cycle. Before ovulating it is tricky to determine if it is safe to have sex unless you are regular.
Remember that the downside of natural family planning is that the odds are higher that you will get pregnant. Also it does not protect you from STDs.
Don't forget abstinance! :) You asked...