The "would you like fries with that" idea is a great one. When a user adds something to the cart at my site, a small pop up window appears asking if the customer would like to see related accessories. If they click "yes", they are taken to a page with items that are directely related to the item that was placed in the cart. If they click "no" they proceed to the checkout portion of the site. This seems to work very well at driving multiple sales.