Try simple breathing exercises.
When I was an archer, I used to do the following exercise occasionaly between ends, especially in the later stages of a long day of competition or when the pressure began to build. As soon as I would open my eyes after completing this exercise, I always felt more calm, alert, mentally refreshed, and focused. I also noticed a significant improvement in visual acuity. The target would look both brighter and sharper.
Sit down with good posture, close your eyes, and take approx. ten slow, steady, even breaths. Focus on pulling the air in with your diaphragm: imagine you are filling your lungs from bottom to top. Inhale and exhale should be exactly the same duration with a short pause (maybe 1sec) in between. Fill and empty your lungs at a perfectly steady, even rate. Fill and empty your lungs as much as completely as you can with out upsetting the even rythm of your breathing. Each breath should be exactly the same in every way. Think of absolutely nothing except your breathing; feel the air passing through your nose, throat, and trachea, then filling your lungs from the bottom to the top and emptying them from the top to the bottom.
If you want to go a step further, continue breathing at this pace with your eyes closed, but now shift your attention to COMPLETELY relaxing every muscle in your body, from your head to your toes. This works better lying flat on a hard floor, with a firm cushion under your head, but you can do many of your muscles sitting down too (maintain good posture).
