As we have mentioned throughout, the biggest thing to worry about when you're riding in cold weather is the wind. The problem with wind is that if there is a gap in your clothing, the wind will force its way in there and gnaw away at your warmth until your teeth are chattering and you totally regret ever rolling that dang bike out of your garage. So, in order to avoid both the wind and the regret, you'd better make sure you cover every inch of yourself with some sort of clothing or another.
One of the most common areas where gaps occur on a rider is around the neck, and because your jacket will extend only so far up and your helmet so far down, there are bound to be some spots that don't get covered. That is where a good scarf or balaclava will be a lifesaver. By wrapping the garment of choice around your neck, you're sure to keep the wind at bay. If you want to go the extra mile (and since you're riding your motorcycle, you might as well) go ahead and wrap your scarf around you head as well as your neck. Doing so will help you out twofold, because you'll not only keep the wind out of the gaps around your neck, but also, you'll provide an extra layer of warmth inside your helmet.