I read about some new tools available for working with foamboard on TMP (The Miniatures Page, an absolute treasure trove for miniatures gaming of all kinds: www.theminiaturespage.com).
The company is called Foamwerks [www.foamwerks.com] and their product is supposed to be available at Hobby Lobby stores. I am planning a check this weekend if I can get to a store.
The most interesting tools to me are the clips the have for butt joints on the foamcore. Both end to end and T-junction joints can be made, and it holds the foam perfectly while your adhesive dries. I'll probably be buying a pack of these if they are in stock. If not, I may even special order them.