I just had some custom cushions made for my Bobcat's side dinette and rollover bed. When I picked them up, I noticed that the foam was really in the cushion covers tight. I told the upholsterer that I plan to occasionally remove the covers to clean them and was wondering how I would ever get the foam back into the tight covers. She told me that there was a trick to it. She said to simply put the foam into a plastic bag of some sort (like a plastic grocery sack) and then the foam would slide into the cushion cover very easily. Once the foam is in place, tear the plastic bag from around it and voila, it's in place.
I haven't tried this yet but it makes a lot of sense and I thought I'd share it with everyone.