A couple of ideas that have worked for me...
When you are standing, be sure to flex your knees a little while you are waiting for a shot. Rock back and forth just a little too, this keeps you relaxed and lets blood keep flowing to your extremities. If people tense up they restrict blood flow and make it more likely to start shaking, etc.
Concentrate on breathing too, don't starve your brain of oxygen right before a big shot!
Especially as it gets colder, dress appropriately in terms of staying warm. If you are already on the verge of the shakes because of cold conditions and Mr. Big shows up just then, that may be enough to push you over the edge so to speak.
Other than that, experience is the best help in these situations. I never really got that nervous, but I would rush things and not take the extra 5 seconds to be sure of my exact point of aim. Someone once told to aim for a hair, not a deer. Doing that has helped a bunch.