There is one school of thought that says for someone with a lot of time left to contribute more money and a low bond allocation (say, not more than 20%), that you should keep your bond duration long and the volatility of bonds close to stocks. In this case keep duration greater than 10 or even 20 years, but realize that these bonds will sometimes crash just as hard as stocks can do (and maybe for a lot longer), and that you are in it for the hope that the crashes will occur at different times. This is what I do.
Another school of thought is that you should match the bond market's duration, which is easy to do by owning a total bond fund. A lot of people do this, and you will have a lot of cheerleaders to help you stay the course.
Some people (Bernstein, Buffet?) say to keep duration short so you do not suffer losses on your bond side. This might make sense in many cases, but bonds with short duration seem to be losing to inflation recently and you would be better off with CD's, IBonds, or maybe Redneck Bank.