Be smart in your defense for optimal pain.
Many self-defense classes speak about the soft spot target areas where you would want to strike an attacker. A soft spot is an area on our body where damage and pain can be done easily and quickly.
Striking these spots are not a sure fire way of getting away from someone, but striking these spots over and over again will cause the most pain and will increase your odds of escaping.
1. eyes : either with your thumbs simply pushing in or an object jabbing into the eyes
2. ears : cupping and boxing the ears, or pinching, twisting, and pulling the earlobe
3. temple : a knuckle into the temple, or an object jabbed into the temple
4. throat : pushing into the adam's apple with your fingers or fist
5. groin : kick, grab and pull, or knee
6. foot : stomp with shoe
These are just six places which can cause grief to an attacker. Remember, they are looking for an easy target and don't want to be hurt themselves. The more problems and pain you cause them the better for you to escape the situation.
Please remember, one strike will most likely not cause enough of a problem for your attacker. Be prepared to strike them 4-5 times and to go 100% against them.
This site has a great poster for soft spots and explains a little more about why each spot is advantageous.