Sign up for the Amex Platinum card for yourself and as many supplementary cardholders (I think 3 is max). Wait until you receive the cards in the mail then call up to cancel. You won't have to pay the annual fee, but you have the card! The cards will work fine in all American, Continental, Delta and Northwest lounges. I've been in many airport lounges throughout North America and never had a problem getting in. They swipe the card, it beeps but even though the account is non-active they welcome you in.
If you're feeling adventurous you can also sign up for the additional Amex Platinum benifits before you cancel (Avis, Hertz, SPG Gold etc).
I've been doing this for 3+ years, it really works.