First do a google search for GRUNGE or better yet, a DeviantART search. I did, and found this on the first page of results:


At this point, you should have the map drawn. This simple map has a border layer and a land layer. The ocean was the background layer.

Here I've added 2 grunge layers. One for the land, and one for the ocean. For the land, I added a layer mask to mask out just the land. Check out the blending settings also. Experiment with different styles - depending on the hue of your land, the settings will have different results.

Then add some layer styles to give the map some flare.

And you end up with this. The grunge layer is not overwhelming. Just enough to make the map a little more realistic.

Good Luck!
[DOWNLOAD ZIPPED PSD HERE]
http://www.rjbeals.com/files/grunge/grunge-00.zip