One of the ways to become less jealous is to actually learn to visualize and imagine the relationship loss. Most people are very jealous because they think losing that relationship would be absolutely catastrophic, the end of the world. A lot of times like with treatments of any sort of anxiety or unfounded catastrophic beliefs we actually have people imagine what would it be like if that event took place. What people find is that loss of the relationship is not catastrophic. It's something they can survive and most will.
By imagining that loss while relaxing yourself, you become desensitized to that loss and often become less jealous in the long run. Another way to become less jealous is to simply understand that most people do have attractions to other people. That's just our nature. We see attractive people all the time. We are emotionally and mentally unfaithful all the time, however, that doesn't threaten the relationship. Most people are committed and if you are comfortable in the commitment then you realize that just because your partner's talking to somebody else or has a feeling of likeness toward someone else does not mean that they're going to leave you. It's nature. It's human nature to like other people, to engage in conversations with other people and believing that your relationship is special and authentic is one way to become less jealous.
Another way to become less jealous is to develop a sense of trust in your partner. I think a sense of trust is essential to a good relationship. Some people are naturally paranoid or mistrustful so that might be an issue for an individual therapy. So examine what causes this mistrust, what causes paranoia, what causes this hypervigilance and a lot of times there might be psychological childhood factors that deal with inability to trust, inability to believe that your partner is faithful and your partner wants to remain with you.
Another way to become less jealous is to stop comparing yourself to others. There will always be someone who is more beautiful, younger, richer or more successful; therefore, those kinds of comparisons are always negative. You have to remember that you're unique and authentic and your partner fell in love with you with all of your good qualities and bad qualities, with all of your uniqueness. Therefore, avoid those social comparisons. Remember, you are your own human being and your partner loves you for all that you are.