I have a couple of buttons of which I modified how they look. I have set them as flat buttons with a background and a custom border so they look all pretty and nothing like
There is another way which works well for flat styled buttons. Don't use buttons but labels. As you are completely replacing the UI for the button it does not matter whether your use a button control or a label. Just handle the click in the same way.
This worked for me, although not great practice it is a good hack and as long as you name the button obviously (and comment the source) other coders will pick up the idea.
Ryan