I saw a new device at the 2011 PGA Show in Orlando called the PRO-HEAD Trainer. Rankmark, a Leading golf product testing company,said in its February issue that it is a "Trainer that works." The PRO-HEAD offers two benefits: learn to keep your head behind the ball through impact; maintain your spine angle.
Several leading instructors, including Martin Hall, have endorsed the product. May be worth trying for less than $200.