riding the commodity bubble

The commodity market has been the new "growth" point after all the turmoils in the financial sector.
Among the EFT funds, USO alone has risen 15% since March.
The question is, when do I cash out?

In the long run, USO will still rise, but it seems to have reached a short-term correction zone. There is clearly a bubble in the oil price. What message would burst the bubble and send it down-hill? Maybe another OPEC meeting? Maybe a senator bill to raise the tax at the pump. (Just find that John MacCain had a bill to suspend gas tax this summer, which will only increase demand and thus increase oil price.)
Closely tracking the price, I can feel the fear factor: there is always a wave of cash-out in after-hour trading that send the price down for the next-day's opening. Low and behold, the price will climb up again and reach new high during the next day's day-trading.

It's always tricky to time the market, even if you know the trend.