State your boundaries. Share your boundaries with your parents, friends, and anyone whom you are dating. You must be committed to maintaining your boundaries.
Avoid dangerous situations that could make it harder to abstain. This could mean not attending parties that serve alcohol, avoiding a person that is tempting you to be sexually active, and avoiding places such as bedrooms or places with a lack of proper adult supervision.
Firmly say, "No!" The word no is clear and to the point. You do not have to be rude, but you must be clear vocally and with your body language.
If all else fails you can exit a situation. No matter how close you come to crossing your boundaries you can always stop and exit.