Take a moment to just breathe.

Close your eyes, take a deep breath, and let it all out slowly.
Focus on your breath, follow it in and out.

Do this for a few minutes and really try to focus on nothing but the breath.
This can be hard, but the more you do it, the better you get at clearing your mind.

This can help calm your mind and body and help you stay in the present moment, rather than letting your mind wander off to places you may not want it to wander to.