BHP Billiton ( BHP) has had a stellar run in the last six months, climbing more than 20% over that period while the S&P posted performance that clocked in at around half that. But BHP looks like it's finally rolling over. Here's what to watch for in this commodity giant. BHP is currently forming a head and shoulders top, a price pattern that indicates exhaustion among buyers. The head and shoulders is formed by two swing highs that top out around the same level (the shoulders), separated by a bigger peak called the head. The sell signal comes on the breakdown below the pattern's "neckline" level, in this case at $76. Momentum, measured by 14-day RSI, has been in a downtrend since all the way back in December, when BHP first started showing signs of weakness. Since momentum is a leading indicator of price, it adds some significant evidence that BHP could be in for lower levels. On the move below the neckline, I'd recommend keeping a protective stop at $77.