Shower, get dressed and get out of the house every day. This was the hardest for me in the dead of winter. It was cold out there and I had plenty to do inside where it was nice and warm. In fact for the first time in my life I developed a Vitamin D deficiency from lack of sunshine.
Exercise. Even if you weren't doing it before, exercise now. Go to the gym. Play tennis. Ride a bike. It can be as simple as walking in the park.
But get out. Create excuses for yourself. And I don’t mean just taking the elevator downstairs to get the mail and chat with the doorman. Even if it is just to indulge in a Starbucks down the block, get out.
Lastly, try not to drown yourself in alcohol. That will only serve to keep you lingering at the edge of depression.