I agree with the others. It's hard (for me at least) but I do it as long as I can. Things that have helped are goals that are slightly out of reach. For this year, Iu wanted to ride 2,000 miles commuting. this will take me until Thanksgiving-ish to complete (although it might be out of reach due to lots of early season rain). When you are close to your goal, and only need a few more rides, it's easier to get out there when the weather is bad. Also, starting early so that by the time the cold weather hits commuting by bike is your default method makes a huge difference.