I very rarely get jealous; not something I intentially do, just part of the way my brain ticks. If someone doesn't want to be with you, then don't want to be with you. No point upsetting yourself over something out of your control.
I think its human nature to flirt though, so I've never felt the need to get upset by that, which is good because I tend to do it myself. I just figure that if some woman makes a play for my man, and he takes her up on the offer, I am better off without him and walk away without a backward glance.