1) It may sound trite, but how about you believe your partner?
Yes, take them at their word. If they do lie to you, then they are not making a fool out of anyone but themselves - remember that. It's been said that trust is the cornerstone of any relationship. It's very insulting for your partner to have you always doubting their word or decency of behaviour. Constant questioning by you can even be as destructive as having an affair in the long run.
You'll still distrust your partner for a while (out of sheer habit), but find the strength to start acting as if you believe them. If you've been checking that they really were where they said they've been, then stop doing that. When they tell you they love you, believe them.
2) Easier said than done, but stop comparing yourself to others
Some (not all) jealousy is driven by low self-esteem. "How could they love me? I don't understand how someone like them could be attracted to someone like me!" We none of us are supposed to understand exactly why someone loves us. Does the Mona Lisa painting know why it is so valuable? Of course, you may be able to appreciate attractive qualities in yourself, but consider this:
There are better looking, richer, funnier, smarter, younger people around than just about all of us, but these are qualities of a 'product'. If he or she loves you, it will be because of an extra, indefinable quality you have that they couldn't even explain - some deep part of your humanity they connected to which transcends looks, youth, wealth, and so forth. Some of the most loved people in history have been well down the list when it comes to looks or wealth. Stop trying to 'work out' why they can possibly like you.
3) It might be a terrible thought, but be prepared to lose them
I said that not all jealousy is driven by low self-esteem; and that's right. People with quite high self-esteem can experience intense jealousy if they tend to feel they themselves must always be the centre of things. People like this tend to look at other people as material property. And maybe they just don't want to share that 'property', even as far as letting their partner innocently smile or socialize with another person. Perhaps as a kid they were a little spoilt.
But people are not objects or toys to be constantly guarded. To love someone properly, we need to be prepared to lose them. What? Am I mad? Sounds like it, you might think (and I do have my moments), but hear me out.
Anger, fear, and jealousy drive out love; and love needs a strong dash of fearlessness to flourish. Okay, so you fear losing your loved one to someone else (and possibly fear how this will make you feel about yourself). If you must keep using your imagination, use it to imagine the 'worst' happening and you still being okay; not just surviving, but thriving in this imagined scenario.
Fantasize about how well you'd react, how whatever doesn't kill you makes you stronger. Write down 10 positive ways you'd like to respond and how you'd build your life up even better if this relationship were to end. Fear is much greater when we feel that 'all our eggs are in one basket'. Don't build your whole life around any one person. "How can I live without you?" is too daunting - really imagine how you would, if you had to, live without this person.
But don't leave this list lying around to be found by your partner, as this may start them feeling insecure. :-/
4) Don't - just don't - play games
Jealousy is excruciatingly uncomfortable. People sometimes try to make themselves feel better by trying to get their partner jealous. Don't do this. Flirting with other men or women all the time in front of your partner; constantly saying how attractive, fun, and witty someone you work with is; and going out of your way to talk about past lovers just demeans you and won't make either of you feel better in the long run.
This isn't to say you have to pretend that no other attractive people exist in the world, but you can acknowledge this without using it as relationship ammunition. If your partner is ever unfaithful to you, that is a reflection of them, not you; and if this were to occur, it's better that they don't have the 'ammo' to turn around and say: "Well, you were always talking about..." or "Can you blame me? Because you were always flirting outrageously with the auto repair man (girl who works in the bar)..." Keep your dignity long-term and ditch the game playing.
5) Stop confusing make-believe with reality
Jealousy, like many psychological problems (from hypochondria to paranoia), is driven by the destructive use of the imagination. The imagination is great...if you use it for your own benefit, not if it messes with your mind. Stephen King has a stellar career from making stuff up and writing about it. But he distances himself (thankfully for him!) from stuff he creates in his head. He doesn't believe everything he writes is real just because he imagined it. Right now, I can imagine an alien invasion headed right towards Earth. I can vividly 'see' the pesky aliens about to land the mother ship in my local park, but I don't believe it.
Stop trusting your imagination so much. Think about it:
Your partner is home later than you thought they were going to be.
You start to imagine them having an intimate drink with that handsome guy you saw working in her office or that luscious sister of his new gym partner you happened to see one time.
You become angry, upset, frightened - without having any evidence that what you imagined is real.
They come home and you react 'weirdly' by being very cold or you have an outburst of anger toward them.
They become defensive and angry back in turn.
I recall seeing a YouTube video of a dog becoming very angry - with its own leg. The more its leg moved, the angrier it got with it - not realizing that it, the dog, was moving the leg. We laugh when we see a dog do this, but psychologically people do a variation of this all the time.
When you stop getting emotional just because you've imagined something, you'll take a hefty step toward regaining control of that jealousy.
6) Lengthen the leash
Okay, since we're talking canines, here's another dog reference. Start relaxing with lengthening the 'leash'. If your partner wants to spend the weekend with his or her friends, let them. Keeping them 'imprisoned' will only build their desire to escape your possessiveness. Let them have their freedom (and no, this is not the same as letting them walk all over you). If you are out with them, let them chat to their attractive colleague (bearing in mind that they may not find their colleague as attractive as you imagine). If you suspect your partner is trying to make you jealous, then short circuit this by relaxing about it; but how?
7) Use your imagination to make you feel better, not worse
Try this exercise:
Close your eyes and relax. Now think about the type of scenario that makes you the most jealous. Is it knowing your partner is out and you imagining them with someone else? Is it seeing them talking and laughing with someone else?
Now, breathing deeply and focusing on relaxing different parts of your body in turn, just imagine seeing yourself looking calm, relaxed, even disinterested in that type of situation. Because ultimately in life we only have ourselves to answer to, and you can only truly control yourself. Visualize your partner doing all the things that made you feel jealous and see yourself not responding with jealousy, but rather with calm detachment. The more you can do this, the less jealousy will be able to mess with you.