Despite what the pundits say, average annual returns of 20 percent are not an inalienable right. The past several years have proved to be remarkable, but that doesn't mean the bull will run forever. But it doesn't necessarily mean the market will crash either. To paraphrase Yogi Berra: Investing is 90 percent mental--the other half is monetary. Babe Ruth had one of the finest batting averages in history, and even he struck out sometimes. So if you expect your portfolio to swing for the fences every year, you'll probably end up in the minor leagues. Give your portfolio a break by periodically rebalancing your assets.