I drive on days when the weather is cra*ppy. That way, I appreciate riding on the other days. I usually end up riding 3-4 days/week. I think I would end up dreading my commute if I rode on days when it's pouring rain, sleeting, ice storms, etc.

Another trick is to reward yourself. Do you have a coffee shop or restaurant you can stop at occasionally as a reward? Unfortunately, I don't on my commute unless I really go out of the way. There is, however, a nice ice cream shop, but I don't crave that much on cold winter days.