Make sure your shoes aren't too tight or have any pressure points. If you're on the bike, curl and flex your toes and pull up if you're using any kind of foot retention.
Sometimes I extend my legs and stretch and shake them one at a time. If it gets real bad you can take your shoes off and massage your feet.

Don't forget about hydration. If you get cold dehydration will make it worse. If you're working hard in the cold you still lose hydration through breathing even if you don't seem to be sweating a lot.