There is several forex sites that offer short/long on fiatcurrencys/btc nowadays and last time i looked a site like bitfinex for example had the possibillity to lend usd that one could buy btc with internally on there site and then you have the obvius option if you currently have assets in the form of bitcoins you can always move those temporary to the other side of the fence, sell them for usd/eur or simular on some exchange and rebuy more btc if the btcprice goes down.