List reasons why you want to get back into this relationship in the first place. Is it because you love this person and want to try to make it work again? Or are you lonely, and he is comfortable and familiar? Many people will get into a relationship with a familiar person when they are lonely or feeling vulnerable or when all their friends have coupled up and they are singled out.
Take the relationship slow. Start out with coffee, and talk about how each of you has been since the last time you saw each other. Try not to rehash the breakup or the negative points of the past if you really want to start over. Blaming the other one for the breakup, or accusing her of ending the relationship unfairly is not the way to start a new venture.
Write a list of the reasons the relationship ended the first time around, and think about each one. Have those situations changed? Maybe you broke up because he didn't have a job and now he is employed. Or maybe you ended things because at the time she didn't want kids but now she does. If things haven't changed since you last spoke, you can be sure you will probably not last the second time around.
Make a commitment to start anew. Let go of old hurts and past fights in order to start fresh. Date again as if you were meeting each other for the first time. Jumping back in where you left off deprives you both of that meet-a-new-person rush you get when you start dating.
Hold off on any physical affection until you are both sure this is what you want. Having sex too soon might turn the relationship into a strictly physical one, which will leave one or both of you hurt.