Do your pedals spin freely (without engaging the rear wheel) when it gets really cold? We have the solution: Morningstar 'Lectric Lube. This waterproof grease maintains its workable consistency down to -49 degrees F. Here's a quick overview of the winterization process for a Shimano freehub.
Remove the freehub body from the hub shell.
Remove the seal from the back side.
Remove the dust cap.
Install the Morningstar Freehub Buddy.
Purge the old grease out.
Decide if you want to try to replace the old dust cap or install a new Morningstar dust cap.
Install the dust cap.
Re-assemble the hub. Done.