Sometimes feelings of situational or other depression (feelings that aren't associated with serious mental illness diagnoses, which generally need to be addressed with medication and/or therapy) can be wiped out in the simplest of ways. Methods so rudimentary that if they were suggested to a person who is depressed, that person might respond with a roll of their eyes.
The fact is, when most people are feeling low, they are also feeling unmotivated and tend to wallow for too long. They lean more towards laying in bed or on the couch, immersing themselves in computer activity, music or the television for extended periods. Often, depressed people must force themselves to make a move in the right direction. Occasionally feelings of dysphoria may trump the strength to get moving altogether.
Next time you're feeling low, push yourself as hard as you can to do one of the things on this list, or something similarly stimulating that produces endorphins or creates a mind shift. At various times I've tried nearly every idea in the linked article to cure a case of the blues, and I'm rarely disappointed in the result.