Just leave them out of your vest in the "inflated" position until the night before or morning of your hunt. It won't completely prevent a bit of memory from the rolling/folding, but it surely will help. I have a delta foam hen thats exactly 10 seasons old now. Foam is actually getting thin in some places, and she has a few pellet holes in her now (which is a good thing ). She doesn't retain much folding memory anymore, so over time, yes they do tend to stick to their shape better.

There is an aftermarket decoy stakemade by buckwing I think that looks like an umbrella frame and is designed to spread/inflate the body of a foam decoy. If all else fails, that should help some too.