Let's say you are playing a game. Then suddenly, you make a bad move and e.g. blunder material. How should you react afterwards? What mind-set should you have? Can you even benefit from the blunder?
After a blunder, it helps to look objectively at the position once again and forget the history. You might have been better or worse; but that doesn't matter now after the blunder. What matters is how you go from here.
There could be many possibilities of salvaging a draw or even snatching a win by making the position very complicated and causing your opponent to blunder (I sometimes jokingly call this a "counter-blunder"). The least you want to ensure is to not make it psychologically easy for your opponent to win after your blunder. Make it difficult for the opponent. Play good moves. Do not be gloomy lest you blunder again.
Yes, there are times when you can even benefit from a blunder. A blunder may often give a player a kind of "license to thrill"! Now that you've blundered and your game is lost, you've got nothing more to lose! So why not embark on a nice sacrificial attack on the opponent's king (for example) and see if the opponent cracks under pressure? (A friend of mine actually tried this after blundering a piece and he won the game. Too bad I don't have the game).
Excellent points by Wes. I'd just like to add that, like with after any move, it's a good idea to look at the board as if you just came to the position in a puzzle. We often get so caught up in our plan that we miss better strategies that pop-up, or we don't see an opponents win.
If we try to take a fresh look at the board after every move we're likely to spot these things. This is more important after a blunder. Rather than focusing on the loss/mistake we should be looking for the best move on the current board.
It's also worth mentioning that a blunder, even a big one, doesn't always lead to a loss unless at really high level play. I'm currently rated around Elo 1,400 and I can't count the number of games where either me or my opponent have made a blunder but then gone on to win the game (usually through a similar blunder from the opposing player).
I'd say play like the computers do, i.e. just look for the best move in the ensuing positions and continue to play tough. A little aggression wouldn't hurt either, as another respondent has indicated. Put the pressure on and give your opponent the chance to blunder in turn. It's happened. Don't by any means just trade away pieces into a losing ending, which I've also seen happen.