I disagree with some here. When its down around the mid 30F range, I let my bike idle a minute or two. Then I shut it off, put it in first, pull clutch in and restart. Now I am ready to ride. I do this to warm the clutch up a little and avoid the big clunk from shifting into first with a cold and draggy clutch pack.
Idling a minute or two harms nothing. I do agree that its best to ride at least a half hour or more in the cold and not do very short trips. I also like synthetic oil for cold weather use. In the car I do the same thing except the turn off/restart step.
"If its not broke yet, it can still be fixed"