I used to be really jealous but then I realised that I was going out, chatting up several girls a night, sleeping with a few different girls. It was something I had to just "get over" - If I'm allowed to play the field and won't commit to a particular girl then I can't expect her to do the same.

That said if you are committed to a girl and still feel jealous a - remember you trust her and she isn't putting it a bout. If you can't trust her, bin her off and move on,
b - Talk to her about it if it's over a particular aspect e.g. dress, 1 guy, drunken behavior, etc. If she isn't receptive bin her off and move on.
c - There's not a lot you can do to stop it, so why worry, if she does just bin her off and move on.