Haven't read all replies but here are my tricks for doing long cold dives.
Breakfast! Everything starts with a good breakfast and a good warm lunch. Proteins and fat. Eggs, bacon, avocado, OJ, bread and butter. Your body needs fuel to keep warm.
To keep extremities warm, they need bloodflow, AND that the flowing blood is warm. IE; keeping the core warm is significant when trying to keep extremities warm. Cold hands might be first sign that you are actually slowly cooling down and the body is compensating by constricting periferal capillaries, leading to even colder hands. SO... More clothes on the body. Use wool. Don't ever use cotton! Find your inner sheep!
Make sure you either have a properly cut cuff/cuff seal that will not constrict around your wrist, or that you use a decent ring-system on your suit.
Wear properly fitting gloves. If the fingers on the glove are "springy" they can constrict bloodflow and fingers feel cold.
Keep head warm. While the myth about most heatloss coming from the head is debunked in normal cases, diving a drysuit we normally use neoprene hoods that will cause heatloss from head/neck area.
However... Exept for the point about wristseals and gloves being circularly constricting, proper food is the biggest factor for keeping warm form me.
