On the chart above, notice that over the past month, there have been 6 distribution days in this weak rally. William O'Neil explains that this sort of behavior normally precedes a market decline, as it indicates that institutional investors (who create volume) are selling. I think it's important to note that this distribution has occured in a fairly small price range, as this shows that these sellers have been capping the bulls strength. The last time I noticed this sort of distribution was at the beginning of the May-->July downtrend.
Also, notice the head and shoulders topping pattern. I would consider shorting on a daily close below the neckline on higher volume, with a stop above the recent high.