When I am feeling stiff, a few deep breaths always help. I also like to focus on the movement of the horse and forget everything else. Just breath with him and let his movements tell your body how to move. (sounds cheesy, I know) but it works.
I sometimes stretch a little before I ride, do shoulder rolls, and try loosen up my body. My instructor used to have me lift up my toes a little, then let down, and rotate my ankles a little, it seemed to help. Some folks drop out of the stirrups for a few minutes too, and let the legs hang. Try a few deep breaths and calm your mind.
Definitely do stretches. Strech your arms and your legs and then before you ride, jump up in the air and shake it out like a crazy. You might want to do this in the bathroom though cause it looks rather strange.