Go to bitcoinica and buy a negative amount of bitcoins. then sell them if you think the price is low enough. profit. OR the price could go up and you would lose money.
Isn't this the same as if you put in a positive number and hit sell? It shows up as negative on your trade list and you are effectively "short". Then you hit liquidate at your profit percentage goal. Then you learn how to get crafty with the Place Order button and have it do that stuff automatically and set a stop loss so you don't get a margin call or lose big.