To defend yourself against an attacker that grabs your arm, their thumbs are always the weakest link. When they grab you, asses where their thumb is, and pull away sharply against their thumb. If the attacker is bigger, use both your arms. Lock your hands together tight, and use the strength in both of your arms to pull away rapidly. If you have the chance, run. If not, yell "NO!" when the attacker grabs you. Practice this at home with other people, but make sure that they will actually try to hold on to you, this will help you become used to this move. Practicing can build muscle memory, so that eventually, this break away will become natural for you.