Funny someone should ask this just around now, although I suppose since it's winter it's not surprising. But I just
spent Christmas up in Seattle, from San Jose, CA and took my bike the whole way. SJ->Seattle via 101, Seattle->SJ via I-5.
My K1200RS has heated handgrips, I also brought an electric vest. Temperaturs up ranged from 40-51 degrees, since it was the coast, along I-5 on the way back it was around 29-45 degrees. Heavy rain up, dry as a bone down. Here's some stuff I learned in no particular order:
o As long as the temperature's lower than around 45 degrees, you will ALWAYS get colder the longer you ride, period. Layering makes no real difference as far as I could tell, the wind just leeches heat from your body no matter how much you try to keep warm, and your body temp just keeps dropping as long as you're on the road. Frequent breaks are a must.
o An electric vest is *critical*. By keeping your torso warm your extremities stay warmer, although see the note above -- It prolongs the inevitable, doesn't prevent it.
o Triple-layer gloves seem to work best. I wore glove liners, Held gore-tex lined gloves, and aerostitch 3-digit rain covers over the top. The cover helped the most, acting as both a wind and moisture guard. Again, though, your fingers get cold no matter what, even with heated handgrips.
o Make sure your boots are loose, even over the thickest socks you may wear. If they're comfortable in summer weight socks, they're useless as cold-weather gear. You need an airspace in the boot for warmth retention, and you need movement to keep your toes from going numb. I wore single-layer expedition-weight wool socks under my touring boots which helped, but they were a little too tight. I think I need two pairs of boots, if I want to tour in both summer and winter.
o Wetness=Cold. Anything damp will suck heat from your body in a matter of seconds, and that's not an exaggeration. I wore an Aerostitch suit that did a fine job keeping me dry, but my boots got wet enough on one day for a little moisture to get through to the sock. It felt like a damn ice cube!
o Your head doesn't get cold. I wasn't expecting that.
o In general, triple-layer clothing works well, as I mentioned with the gloves. I wore long underwear, the electric vest as an intermediate layer, thick fleece, and the 'stitch over all of it. Still got cold, just slowly.
Those're the most important ones, I think. Hope it helps!