Rear Bear Hug

If someone gives you a bear hug from behind, you want to drop your weight to prevent them from picking you up by stepping forward with one foot.

Then, if your arms are free, use one to swing past your back leg and put your weight into striking your attacker's groin, a few times, hard. An elbow to his face will then send it to your other elbow and then back to the first. By using left and right elbows in this fashion, if your attacker avoids the first one he ducks into the second or third.

Twist your body in either direction so you can see your target and also to add momentum to your limbs. After volleying their head with your elbows, turn around (their grip should be weaker) and finish up with some palm heel strikes or knee strikes or whatever is necessary for you to be out of danger.

If your arms are pinned when the conflict arises, drop your weight like you would if they were free and strike the attacker's groin until he loosens his grip. You can also grab and squeeze the groin as well.

At that point, bust that same arm forward and up through his hands, freeing it, then swing it from far in front of you and put your weight into a straight elbow to his head. Now you are facing him and can introduce him to a palm heel strike and maybe a few knee strikes.

The defenses against variations of a bear hug are similar because when you are in that situation, it is not the time to analyze what kind of hold you're in. The responses described here are close to what one would do naturally and, because of their simplicity, are more easily executed in a real situation. More simple and effective defenses against common street attacks are demonstrated on Street Self Defense 101.