That's a very common question with many different answers depending on what is causing the situation.  I've been in both situations and have come to realize that people often don't really care about how they see themselves, but on how others see them.  Therefore, low self-esteem, is not the problem, but an over-zealous desire to be liked by others.  Telling her that she has a problem with low self-esteem may lead to lower self-esteem as she realizes that she has more issues than she thought.  Most people respond well to sincere appreciation from others.  Give praise for every good trait and every improvement no matter how small.  Talk about your own faults and failures to make sure she knows she is not alone.  Also, in general, people want to talk about things that interest them, not things that interest you.  Keep the topic on their interests.  
You don't need to put her is a certain position, don't make it a presentation, just have a normal conversation with her, just be a friend.  Try to put yourself in her shoes, try to find out what she really wants in life.  Myself, I have a high desire to help others.  Some people just want to be recognized for their achievements.  Some people want control over their own life.  The possibilites are endless and the only one who know the answer is her.  
Some people want to be lead to their destination.  Some people want to be shown their destination.  Others need to find their way themselves.  Above all, she may just want to tell someone her problems, so don't forget to listen and understand.  I've just tried to condense a entire book of suggestions into a paragraph, I hope it helps.