Is this all the time or just in a show situation?
If it is all the time, then I would suggest just riding out hacking for a while, remember that this is meant to be fun to start with, so go have fun. If you stop concentrating so hard on being perfect it will help you to communicate! Remember dressage is a dance, but it is also work, the key is to look relaxed while working hard.
What has helped me to some extent is taking up physical fitness, I find the more I get used to getting my body to carry out Zumba moves, the more I can relax while doing it, and that is carrying over to my baby steps into Western Dressage. So maybe try something else to help get the body responding. Oh and I also spend evenings sat on my gym ball, practising using my leg, while not gripping up or tensing.