Here's a quick fix I did for my fireplace this spring since it's sure not going to see any fires with the kind of weather we are having right now!
Did you catch it? Not a big change, but just enough for the fireplace to not look so dirty. I bought a mirror on clearance and laid it on top of the fireplace grate with some of my pillar candles. I like the reflective surface and the glass pillars much more than a dirty grate!