Plan a vacation (for March)
Getting out of town to somewhere with sun is an essential survival tool, even if it’s just a weekend on the eastside of the Cascades, where the sun shines more frequently. Ideally, you go to Hawaii, but that's not always in the budget. The key (should you only be able to afford one such trip) is to do it in late February or March. The worst weather is actually in November or December (and if you can do two trips, take the first then). But at that point, you still remember the sun of early October. By the end of February, you've convinced yourself you might never see sun again. Then you come back, revived and able to make it until spring.
Load up on vitamin D
Take it. Lots of it. Talk to your doctor about how much, but since it's absorbed by getting time in the sun, nearly everyone in Seattle is deficient.