ALL of the above on gear, especially liners under mittens, keeping the core and head warm.
One other thing from cold weather training ( e.g. Camp Drum, Mt Washington )and too many winter mistakes : eat carbohydrates. You use close to 300 calories/hours cutting hard; they need replacing. Munch all the time in the cold. The "energy bars such as Cliff have plenty of carbos and calories to keep the core going. GORP, any chocolate, and a thermos of hot chocolate help ( NO coffee ).
You'll find that your body will also adapt to cold after awhile: what seems cold at 10F after a few days working then 20F feels warm. Don't depend on those chemical warmers or running inside all the time.
Don't let your extremities get to pint of non-feeling; it's harder to get the blood flowing. Take "shake breaks" from grabbing the saw or other gear by shaking your arms and legs up and down BEFORE they get too cold.
And good buddies will give you their stomach to warm frozen hands ----deadly serious.