Ahhhh, freeze the kong!!! solid, like in a deep freeze, and there is an art to stuffing a kong so it takes longer to empty, the kong web site has lots of recipes.
SusywanII--have you tried real bones?? If you have a power chewer it's definitely the healthiest and cheapest way to go. Another trick is to drill some holes in the body of a nylabone and put some peanut butter or cheese into the holes to make them more interesting.