Jumping jacks. Be sure the stand is sturdy.
Merino wool base layer (2x if really cold), good midlayer (sitka fanatic hoodie) and quality bibs and jacket (right now its sitka incinerator bibs and first lite sanctuary jacket). Add a muff, handwarmers, and a good gaiter and hat (the madbomber is crazy warm). I also use sock liners with Vermont darn tough socks under my muck woody elite boots.
Bring a warm beverage and food. Oh and carry come extra natural internal insulation - ie not skin and bones.
Keeps me pretty comfortable.