Plotting your path to investing success might seem like a relatively stress-free exercise when you've got a raging bull market at your back. But not these days. There's a growing fear that the sharp rally in stocks -- the S&P 500 index has nearly doubled over the past four years and gained 15% during the first five months of 2013 alone -- is unsustainable. Asset prices across the board, say skeptics, have been propped up by the easy-money policies of the Federal Reserve and other central banks around the world, and when those unprecedented stimulus efforts come to an end, it won't be pretty. Indeed, those fears prompted long-term Treasury bond prices to drop by more than 7% in May, rattling investors.