Are you in tune with your body? Using Fertility Awareness (sometimes called natural family planning) does not require drugs or devices but you must commit to being one with your womanly rhythms in order for it to work. The key is to not have sex during your fertile period, or use another method, such as condoms, during that time. FAM is 75% effective at preventing pregnancy.
Recognizing the signs of ovulation - noting how much cervical mucus you have, and how it changes around ovulation (mucus gets thin and watery just before ovulation, thickens after) and charting your temperature are the main concepts behind this method. Women also use ovulation predictor kits and chart their cycles for a few months to get a better understanding of when they usually ovulate. From there the ideal days for having sex when trying to prevent pregnancy are the 10 or 11 days at the end of your cycle and the discharge-free days (if any) that occur just after your period.