Don't have sex. If you don't have sex, then you can't get (her) pregnant naturally. If you want to have sex w/o pregnancy, wear a condom to minimize risk of sexually transmitted diseases. Or ask her to wear a diaphragm or sponge, either one w/spermacide. Copper iud & fallopian tube coils are two other non-hormonal methods. Whether you consider these methods "natural" is up to you. Timing is not very effective.