I am giving my mom a digital photo frame that matches her bedroom decor (black and chrome), pre-loaded with family pictures (scanned family pictures from the 60's and 70's and current pictures of the grandkids, Christmas, her favorite vacation pictures, etc.). I think it's a great gift.
A day at the spa or a gift certificate for her favorite hair salon or cosmetics store would also be good options. I find flowers too boring. If your mother is the practical type give her a gift certificate good for ten brushless, full-service car washes or the like.
I find a dress the gift from hell -- she'll undoubtedly hate it and never wear it, plus how could it possibly fit perfectly? DO NOT ever buy your mother a dress.