Here in Maine it can sometimes get fairly cold during early season. You can't change the cold weather but you can dress for it...and you can help prepare for that "stiff" situation (even if the weather isn't frigid) by practicing similarly to how you normally hunt. Do you use a treestand and stand up to shoot? With hunting clothing on (including gloves if you use them), practice shooting from up high, and standing. Do you sit, either in a ground blind or from a treestand? Be sure to include practice from that same position...even using the same seat or stool if possible. Just like the shot itself, how you position yourself in taking it is best engrained so that it feels familiar. It'll help flag any short-draw, clearance or clothing problems as well. Beyond that...form form form.