How do i get over a guy that i want to remain friends with?

well i have to be friends with him really because we have so many mutual friends. i really like him as a person as well and really value his friendship. but it's so hard to hang out like normal when i really like him and i know he liked me once too. now he has a girlfriend and they seem to love each other. this really hurts especially when he still flirts with me and says things he really shouldn't while having a gf. although there's always been this flirtation between us, there's also an overriding feeling of sadness whenever we hang out cos we get on so well but will never be together :( as they say if you love them, set them free - but how can i do this while still being friends?