Yep, I have the same problem. It's because my heels are narrow and the balls of my feet are not. No solutions, other than to wear only boots or shoes with straps.
Actually, that's not totally true. I have a couple of pairs of gorgeous pumps, and while I wouldn't wear them for walking all day, they are comfortable enough for a "sitting day". I bought them a little tighter than were strictly comfortable, and got the front of the shoe stretched out (a lot) with a shoe stretcher.
I don't wear ballet flats, but it's no loss because I don't like how they look on me anyway. They make my feet look ridiculously tiny!