I get the same problem, even in summer. Poor circulation is probably the root cause, made worse in the right foot by having broken most of my metatarsals at one time or other. I get round it by wearing loose-fitting shoes (Northwave have a really wide toe box which helps - previous Carnacs were terrible), and fairly thick socks (Woolie Boolie Meriono ones are ace), and remembering to wiggle my feet around. Also exaggerating the ankle movement for a while helps get the blood flowing again. If it gets really bad I unclip on descents and wave it around. Gets some funny looks but works.