I go out, get a huge cup of coffee, and take a long walk in which I engage myself in talking to strangers. It’s nice to know that I can afford to buy coffee, I have the physical ability to take a walk, and people who don’t know me are willing to talk to me.
I observe my feelings and I wait.
They pass.
There are a few things I do.
Use the good dishes. Even if all I’m having is canned soup every once in a while I’ll use one of the pretty bowls, and the art deco flatware I inhereited. The most frequent manifestation of this is making a whole pot of tea in the pretty china pot and using a pretty cup instead of my South of the Border mug.
Occasionally I paint my toenails ridiculous colors - orange, or sparkly purple. Outside of my home my toes never see the light of day. No one who doesn’t live with me will ever see my painted toenails, but when I see them it makes me smile.
I almost always have a cheer up song. Like Asimovian said, the song itself doesn’t necessarily have to be cheery, but often they are. Bouncing 'round the Room by Phish, Wind it Up by Gwen Stefani, and Drive My Car by the Beatles have all had a turn.
Clean the hell outta my place. Then cook a complicated fantastic dinner.
I cheer myself up in a most dignified way. I drink loads of Diet Dr Pepper and belch loudly, until I laugh myself silly.
Failing that, computer/console gaming until my eyes are square, preferably with my wife tygre alongside.
Sad end of “down”. I usually walk it off or watch some Python.
Angry side of “down”, I go shoot a few rounds of sporting clays.
Sad down: Watch sappy romantic shoujo (girly) anime.
Angry down: Watch American Psycho.
Drive around listening to/singing along with my favorite songs, too many to mention, but “Love Shack” (B-52s), “Rock the Casbah”, “Copacabana”, and any of a number of show tunes get frequent play… Finish some odious task I’ve been putting off (filing stuff, balancing the checkbook, washing floors) - it gives me a feeling of control and there’s one less thing to deal with…Usually just hope I get a good night’s sleep because things don’t look so bad in the morning.
Neat idea. More generally, doing something nice for someone else often helps the giver as much as the receiver.
Failing that, you could always read the Dope… 
I turn my gallows humor and inner critic up to 11 and soon realize being depressed is hilarious, in a dated cliché kind of way. Clowns would marvel at my smiling on the outside, crying on the inside, chuckling on the inside-er shtick.
However, should that fail, I always remember to say a prayer to Charlie Mopps, the man who invented beer, beer, beer, tiddly beer, beer beer.
I turn up some bouncy music and clean the house.
I go to the bookstore and buy my mom a big bag 'o books.
She’s 88 and a voracious reader. She gets around OK, can get her own groceries and do her own banking, but doesn’t have any extra money for buying books.
Mobility issues makes it hard for her to attend her local library’s book sales because the books are in the basement. Stairs are a challenge for her and the library’s high, narrow ones are impossible.
I love buying books. Always have. Now, my book-buying has expanded to picking out books I hope she’ll like. It’s fun and exciting and cheers me up to bring her a bag of books when I visit.