2. You have to forgive each other for whatever ended the relationship.
No hostility can remain for you both to have a healthy friendship. If the end of the relationship was one you can forgive and forget, you must do it before becoming friends (if not, see #5).
3. You must support each other in future relationships.
Your ex-boyfriend finding a new love interest is inevitable, and you have to be okay with it. Jealous behavior is just going to complicate things and send mixed messages. In order to be a good friend you must genuinely want what is best for each other and respect new relationships.
4. Consider a cool off period.
Often times we make the jump from being the boyfriend/girlfriend to the friend much too quickly. Obviously this person was important to you, but when you don't take the time to heal and separate your lives, the roles you play become confusing. In order to move on from one another, you need to take some time to do exactly that.
5. Accept that sometimes being "friends" simply isn't going to happen.
You can't force a friendship, especially if the break up was one-sided or too hurtful to forgive. It’s important to respect your ex's way of moving on and sometimes that means not seeing each other for a while.