Stamping a good image
I too make sure my stamp is inked well. Rub the stamp over the surface of the stamp pad for a few seconds and then tap it up and down a few times. I don't use a stamping mat, just a good solid table and a cutting mat covered in scrap paper.
I recently had the same problem of the middle of my stamp not imaging well, I eventually worked out that my stamp pad actually needed reinking.