You have broken up with her and, after some time, you realized that you want her back. If this is your case, then you need a set of dos and don'ts. You can win her heart again if you combine the elements of time, sincerity and effort.
The first move is to think first. Do not rush into action, especially if the breakup is just fresh. Do not chase your ex through phone calls, text messages and emails. Of course, don't ever stalk her. This will result in her hating you more. Worse, she can get afraid of you-cutting the chances of winning her down to zero.
Next, if you want her back, then do not get depressed. Beware that separating yourself from the world can result to clinical depression. Think positive because you will need all the energy and self-composure if you want to impress her. In parallel to this, don't make yourself a human rag! Do not beg her to come back in exchange for allowing her to do anything to you. Dignity and pride are also essential to individuals in a relationship.
Next, you may want to stay in touch through email or SMS (Just be sensitive enough so that getting in touch does not become chasing.). It may hurt a bit, but it is a way to check on her especially if it is uncomfortable to talk personally. Keeping in contact will help you perceive the "signals" for getting back together.
Lastly, don't be too proud to seek help if you want her back. There are more experienced people in the field of mending lost relationships. Professional love advisers as well as real doctors and psychiatrists write guides or books about getting together again. You need to "read the signals" right and get all the help you can get to succeed in your endeavor.