Step 1: Create a new document and populate it with two new layers.  For the time being, I'm gonna use a white background, with an object layer on top.  I'm not going to go into detail on how I made this shape, but its simply two ovals joined together at 45 degree angles, anti-aliased, and with a plastic effect applied.  You can, of course, use your own interface shapes as long as there is some sort of 3D effect or shading to build upon.

Step 2: Lets get the ball rolling.  Select your object layer and change the layer blending mode in the layer palette from normal to multiply - this will let it show through any layers beneath.  Now create a new layer between the object and background layers, load up a texture, and fill the new layer with your texture.

NB.  You can find textures all over the web - just go looking. You can buys CDs full of them from Marlin & Dosch if you want professional quality.

Step 3: We don't want the texture filling the whole design, of course, so a little trimming is in order.  Hold CTRL and click your shape layer to form a new selection around your object.  Now pick Select > Inverse from the menu, make sure the texture layer is selected in the layers palette, and press delete on the keyboard to trim the excess.   If you used a custom shape and it doesn't show through well, try putting the texture on top of the shape instead (but still make sure the top layer is set to multiply).

Step 4: For more exaggerated effects, consider stacking textures on top of each other.  This way you can manage some effects (especially metal) a lot better than by using a single texture alone.  Most of all, just play around and see what you can come up with.  The little butterfly on the right is hardly realistic, but took only a couple of minutes to design, and looks kinda cute in my opinion.  :)