Jealousy is a form of mistrust? You need to figure out why you dont trust him first. Has he cheated on you, have you been cheated on in the past, does he have a past of cheating, is he a flirt?
Like Mandyloo said, find out why you are jealous. Sometimes it is our own insecurities which cause us to be jealous. Try understanding why you feel that way and when you feel that way.
in my opinion- jealouse girl friends are the best, it just means you care about your boy, but you can't be to jealous, or else he will get annoyed