While contraceptives are traditionally the best ways to prevent pregnancy, I've read that the Sympto-Thermal Method is a popular and rather reliable way to naturally prevent pregnancy. To use the sympto-thermal method perfectly, you must be able to accurately combine and observe three natural family planning methods: calendar (to predict time of month when you are fertile), basal body temperature (a woman's temperature is generally higher closer to and during ovulation), and cervical mucus (if mucus has thinned, this is another indication of ovulation).
http://www.dummies.com/how-to/content/natural-family-planning-using-the-symptothermal-me.html