I have found, from having been in a very long downswing (Turns out I was bad at poker), that taking breaks, reviewing your hands, and having 1 drink helps.
I have also found that playing late into the night, even at lower stakes, while cursing at the computer and having 10 drinks is a bad way to deal with a downswing.
I have found that most of the time when I have a downswing it is accompanied by me playing worse. This is especially true when it is a dramatic downswing over a short period of time. like losing 5 buy ins over 1500 hands. When I have reviewed those spots I found I was not playing my A game, I was calling too much, bluffing too much, and betting for thin value too often. I turn a moderate bad run of variance into a much more losing proposition.
So, the best remedy I have found is to take that break, have a drink (not 10, because then you convinced you can beat that losing streak), and review what you have been doing.