If you know someone who may be in an abusive relationship, you can help several ways:

Tell that person you are worried...    
 Be a good listener and offer your friendship and support...  
  Ask how you can help, this way the person doesn't think your trying to pressure them or get defensive. Encourage your friend to seek help, give them the national violence hotline if they won’t listen to you now.  People who are abused sometimes don't recognize they are in an abusive relationship or are in denial.  They believe things will change.  They may take time to reflect on what was said, and they'll have the number on hand for later.
Educate yourself about teen violence and healthy relationships so an unhealthy one is easier to identify.
Always avoid any confrontations with the abuser...This could be dangerous for you and your friend or family member.
If it is impossible to get out of your relationship immediately, think ahead of ways you can be safer...this means thinking about what to do, where to go for help, who to call for help when that time comes.
Think of how to escape a violent situation if needed...make a PLAN!  Arrange for a safe place to stay, arrange to not be alone at school or in public places.  Include other people in your safety plan...friends, parents, sisters, brothers, neighbors, etc.  
Girls, take some self defense courses. You can find them by looking in your local phone book asking a teacher or asking your local police department.
Let friends or family know when you are afraid or need help.  There is nothing to be ashamed of by asking for help, it's actually a sign of strength!
When you go out, say where you are going and when you'll be back. Then BE BACK by that time. Go out in a group or with a friend, never alone.
This is true especially after you've broken off the relationship and you are afraid there may be repercussions.
Always carry a cell phone, borrow one from a friend if you have to. If one is unavailable, most public facilities have phones. 
Have money available if you need to take a taxi or bus.
Dial 911 immediately in an emergency.
 911 is much faster than dialing your local police office.