We can't help it. Worrying is part of the human experience. We stress out over everything from our workload to the demands of our biggest clients to the well-being of our loved ones. After all, what if something goes terribly wrong? Of course, it rarely does, but that doesn't stop us from expending considerable mental energy on "what if" scenarios.
If you're interested in putting your thoughts to better use, try this technique: The next time you catch yourself worrying over something, ask yourself "Can I control the outcome?"
If the answer is even a partial "yes," I recommend you pick up the phone and get into conversation about it. It's unlikely that you'll resolve your consternation alone. Invite a friend or trusted colleague to talk you down from the irrational ledge you've climbed on to. As long as you choose wise counsel, you'll feel a lot better and make smarter decisions.
If the answer is "no," consider that you're truly wasting your energy on an improbable outcome, and you'd be better off focusing on the present moment. Meditation and mindfulness can strengthen your ability to stay in the here and now.
Lastly, I leave you with a piece of good advice I received some years ago: if you're going to fantasize about the future, at least create one in which you're winning.