ok! where to start? First grab an image you want to add a wear to. I’m going to use this stamp:

Now we want to add a new layer and give it a grey fill, you can do this in a layer mask (on the stamp layer) or on a separate layer. Now go to “Filters > Noise > RGB Noise…” and move all the sliders to 1.

Ok, now dilate the noise “Filter > Generic > Dilate” This should make the noise appear very washed out, so now mess with the brightness and contrast until you get a good mix of 60% white (it will probably be gray) and 40% black. Use the color-select tool and select the gray, and delete it.

Finally we want to add a 1px Gaussian Blur “Filter > Blur > Gaussian Blur” and, if you have not already, move the layer into the Layer Mask. At this point you should have a fairly good wear, and will be able to add some minute changes, but that’s it! and the best part is you can exchange the stamp image without having to redo the wear effect! Have fun!