You only lose money in the stock market if you sell. Until then it's just a loss on paper. The best thing to do in a down market is to hold on to your existing shares (assuming you aren't completely strapped for cash) and/or buy more shares (hey, the price is low).
The only time that doesn't work is if you own shares in a company that may not exist much longer. In which case you can sell and take a loss, or if they go under, you lose 100% of your money invested.