Scott Burns (a well known Dallas columnist and financial advisor) wrote this article about the topic:
Making Commodities Part of your Portfolio - Registered Investment Advisor
He cites Israelsen research which showed that adding a small amount of Vanguard Energy, Fidelity REIT and Vanguard Metals and Mining improved the return of a typical portfolio from 1987 to 2006 without adding risk, and was superior to adding just a commodities portion to the portfolio.
Vanguard Metals and Mining is closed and Vanguard Energy has a $25K minimum. You could use the Vanguard Energy ETF (VDE) and either the Vanguard or Fidelity REIT funds or ETFs. Personally I have 10% VGSIX (Vanguard REIT) and 10% VDE (I follow Scott Burns' 10-speed portfolio).