There really aren't too many methods to short.  That's why there is so much volatility -- no way for people to bet against the price increase.

 - http://en.bitcoin.it/wiki/Trade#Financial

If you look at the beta trial mode that Kraken.com has currently, you can see what they offer.  Coinsetter might be another.    If you have bank (and aren't from the U.S.) you might want to consult BitcoinFund.eu as they might offer short selling to you.