For me, it's faith in God, knowing He always provides for and looks after me. Nothing horrific is going to come my way that I can't handle with His help. Knowing that allows me to let go of the worry about everything.
But even an atheist or cynic could call it Negative Visualization, Not Getting Too Emotionally Invested, or even Not Giving a F***. Meaning, I try to put everything in perspective, recognizing little things like this for what they are (little) in the scheme of things. Realize that many of us are inclined to needlessly worry about things, including things we can't even control, and worrying adds nothing. When I was young I was in that camp, and discovered that it's completely unnecessary and unhealthy.
If you are now inclined to stress over these things, I encourage you to have faith that everything is going to turn out just fine, and watch what happens. And if you insist on indulging stressful thoughts, try this: Take everything to its logical negative extreme, and ask, "What's the worst that can happen?" So say your printed materials are a disaster, and it's all your fault. What's the worst that can happen? Your boss gets mad and yells? So what. Say it happens again next year and they're so upset, they fire you? So what, you will get a different job, probably one that's more suited to you. You've come this far just fine, you're not going to have your life fall apart over the latest micro-problem of the day. Then just let go of the worry, realizing that everything is going to be fine. Life is too important to get wrapped up in those things. Stay focused on the things that matter: God, Family, your health, living the life you want.
And as already mentioned, exercise, relaxing hobbies, and meditation are all excellent tactical ways to take on the physiological side of things.