When I begin to feel stressed from over doing too much in a day or just feeling anxious, I like doing a simple breathing exercise to calm my mind or even my stomach.
I start by breathing in deeply and slowly through my nose and exhaling slowly through my nose. I keep my mind focused on my breathing but not on controlling my breathing. This helps to calm my mind from thinking about other things, issues or anxieties. As you continue you will notice your body will begin to naturally slow down in your breathing and your breathes will become naturally shallow. Don't force anything just continue to breathe as your body directs you.
Try doing this early in the morning to give yourself a fresh start to your day and even when you stomach becomes tensed or you become anxious. It's a great way to relax your body and come back to your true self.
Here's a video to learn how to relax through meditation.