When I feel jealousy, I ask myself why, but with more detail. As in, "what am I afraid of losing if he loves someone else?" or "why do I think it takes away from my value to him if he enjoys another woman's company?" or "what does being upset buy me?" I try to take a step back and examine my jealousy as if I were a scientist looking at a specimen. So, I usually discover, and have to come to terms with, some insecurity or fear I have.
And then I think about the person I love, and remind myself that when I love someone I want them to be happy and feel free and unencumbered by any neediness of mine. I remind myself that I want him to express who he is fully and not keep him small just because I am feeling insecure. I don't want to load him down with my insecurities, baggage, and tears. I remind myself that I want him to be with me because he wants to be, not because he's obligated to be, and if I give him the space to be himself and enjoy life, he will come back to me. I then say to myself, "Why wouldn't I want him to have as much love go his way, as he can possibly get?"
This process of questioning, and then reminding myself of the best ways I know to love him, usually puts me in a good state of compersion. And then, sometimes, it just helps to put on some upbeat music and dance in my underwear, or go do the dishes.