This might sound silly, but I swear I heard this somewhere - something about making sure when you work out that you pull your tummy in - think drawing your navel in to your back. Sometimes people end up pushing the other way instead and I thought I heard that could cause your tummy to pooch out. Not saying that's it, but I always think about that now when I do any kind of core work or standard weight work. I always make sure to pull in the gut.