Depending on the effect you are looking for I'd start by trying a few of these...
1. Effect>Artistic>Fresco or Poster Edges... play around with the settings to get a range of detail. You might try applying this to a copy of the layer and then overlaying that layer on the other with a color effect.
2. Noise>Dust and Scratches
3. This is how I always start when trying to create a distressed look... loosely crumple up a piece of brown paper bag so that you get some nice creases and flaws in it. Scan this in at 300 dpi. Paste it into a layer and resize over the area you are wanting to effect. Then apply it as a color effect like "multiply" or "burn". If it's looking too dark try adjusting the brightness and contrast. This process will yield the most organic look.