I would also be getting out of the stock market if I noticed prices starting to fall and a crash possibly on the way. There are some good and quite simple techniques I would use to time the markets over the medium to long term. I have described some of them in the answer to this question of mine:
What are some simple techniques used for Timing the Stock Market over the long term?
You could use similar techniques in your investing. And in regards to back-testing DCA to Timing The Markets, I have done that too in my answer to the following question:
Investing in low cost index fund — does the timing matter?
Timing the Markets wins hand down.
In regards to back-testing and the concerns Kent Anderson has brought up, when I back-test a trading strategy, if that strategy is successful, I then forward test it over a year or two to confirm the results. As with back-testing you can sometimes curve fit your criteria too much. By forward testing you are confirming that the strategy is robust over different market conditions.
One strategy you can take when the market does start to fall is short selling, as mentioned by some already. I am now short selling using CFDs over the short to medium term as one of my more aggressive strategies. I have a longer term strategy where I do not short, but tighten my stop losses when the market starts to tank. Sometimes my positions will keep going up even though the market as a whole is heading down, and I can make an extra 5% to 10% on these positions before I get taken out. The rare position even continues going up during the whole downturn and when the market starts to recover. So I let the market decide when I get out and when I stay in, I leave my emotions out of it.
The best thing you can do is have a written trading plan with all your criteria for getting into the market, your criteria for getting out of the market and your position sizing and risk management incorporated in the plan.