The most cost effective way to trade gold is through the futures market. For Futures I like Xpresstrade who was just acquired by optionsexpress. If you want to trade spot gold I believe that OANDA offers this on their platform where you can trade it right alongside your forex positions.
Then there is of course the Gold ETF Symbol GLD that tracks the price of gold but tades like a stock. Less cost efficient and less leverage available but probably the easiest way to trade it.
