The most effective method of preventing both stds and pregnancy is:
abstinence
a long term monogamous relationship
condom use
oral contraception

Abstinence is the most effective method of preventing STDs and pregnancy. In any sexual education courses, abstinence is the most suggested form of preventing these things from happening. When a person consciously chooses to make this decision it will always lead for the better good. Some religious groups also practice this method. This is not to deprive a natural human need, but rather to instill a value in individuals, especially the young ones, that there is really no rush to be doing this and the right time will always be waiting.