Hit'em with a hair dryer and they should retake their original shape. That will fix them at home. The key is to not store them folded or rolled up. If you only have them folded up the morning of the hunt, they should pop back to orginial shape pretty quick. Expanding foam sounds like a pain in the butt to me... are you talking about something like that great stuff foam that gets hard or like egg crate mattress foam/sponge foam?

Another thing that I have seen done is to cram balled up newspapers or used giftwrap. If you have a carry bag, then that will keep you from having to fold them up in the first place.. just make sure you leave yourself some room to get the decoy stake through... also makes them less sussceptable to being blown over by the wind.