Rule number 1) Get up and get to work when the weather is crummy. Just hunker down and work like crazy. This means you can take a slow start or an early departure when the days are nice.
Rule number 2) Give yourself deadlines even if you are working on pieces that have not been bought, yet. This helps you to accomplish things to again justify the sunny day enjoyment.
Rule number 3) Do allow yourself to sneak out for a peek of what is to come. This lets you get a taste of the sunny glow to keep you motivated to finish whatever it is.
Really three rules. And I think these rules only apply to people who live above 43º latitude. Ok, maybe only to those of us who live in the ever so rainy Pacific NW. For the record it is really lovely out right now, and the rains are coming back tomorrow so I have to finish the post and go soak up a little Vitamin D.