This may be challenging, especially if there is no longer a spark between the two of you. Or, harder if there is no returned feeling on his part. But being strong and independent while displaying your friendship and affection may work in your favor.
Avoid obsessing over him! Slowly work your way closer to him through a strong friendship. Also, be the independent person you are. Continue with your clubs and hobbies and be a stronger person. Don't boyfriend hop. If you want him back, making him jealous or angry won't work.
After you've proven your friendship and continued commitment to him and continued to develop your own personal life, feel out the scene. See if you still want to be with him. If you decide that he is worth more than a friendship and that you really want him as your boyfriend, follow the next step carefully.
Drop the idea that you are still interested in him, that your friendship is great but that you really want him to be your boyfriend again. See his response. If it is negative, back off and continue developing your friendship. If it is positive, slowly ease your way back into being his girlfriend. Be patient, yet persistent.
Remember that where there were flames, ashes remain! You were together for a reason so if you really think he is the one (for now, at least) win him back with your charm and positive attitude.