Overlayed Texture
If you'd like to add an almost retro popart effect to icons, this extention of my Seperating Layers tutorial is for you! We'll be going from the first cropped image to the final icon below.
First thing, start off with your base. Sharpen and fix it up like you normally do, but don't mess with the colors yet. (I forgot to sharpen mine until the end.)
Desturate it....
...And go to Image-> Adjust-> Gradient Map
Photoshop will take the two colors in your palette, and if they aren't black and white already, you can click to gradient and choose the black to white gradient. Image it like a curved test, it takes the darkest colors and makes them black, and takes the lightest and makes them white, automatically boosting contrast perfectly.
Choose a texture with lots of different colors or designs on it like this one by arisubox. Drag it on top of your base so the darkest parts of the texture are overtop the darkest part of the base.
Now is when you need to look at my Seperating Layers tutorial. For a little refresher for the lazy, go to your base layer and Ctrl-A,Ctrl-C, go to the texture layer, press the quick mask mode button, Ctrl-V, go back into normal mask mode, press delete, and deselect all. Your icon should look something like this.
You can stop now if you like what you've got, but I don't, so I'm going to continue.
By making a new layer over top the original base, but under the texture, of a nice light brown color. I also added another texture on top of the first, a medium opacity one on a multiply layer, because the first one alone wasn't dark enough. My icon looks like this now, almost like a nice sofa. XD
To finish it up, I added the following mask also by arisubox, and some tiny text overlayed with the matching texture.
Remember, this looks best when your image has lots of strong darks, and whatever you want to emphasize is dark. If you're using an image that's all midgreys, or very light, it won't look as good.