Platinum plugs have a colder spark but need to be changed very infrequently. Copper is hotter but only lasts a couple years. I recommend Motorcraft AGSF-32C.
www.bullittarchive.com has tips on this and many other preventitive maintenance tasks...
wow i am impressed.. all of you guys said it right..
use the stock plug. i like too use the motorcraft regular plug. if you go too your ford dealer they can get you the reuglar non/plat verision of the stock plat plug.