Some good advice from Flyflinger. ^
I tend to prefer faster retrievs that result in erratic, side to side swimming action from flies. Even in the cooler and cold months of Sept thru December, I'm usually pulling muskie flies pretty fast.
In dead of winter, I like a slow, jig-like hop on the bottom, but with water temps over about 42 or so degrees, I like to fish 'em aggressively. River muskies (and pike and walleyes) are quite active in cold water. I recommend a long, hard strip/pull that really rips the fly forward and that allows it to stop suddently and turn or sweep to the side, and then drop slowly for a few moments. Then give it another long strip. This erratic, side to side swimming action, I'm convinced, looks like an injured sucker or prey species, and can really trigger fish.