Before you start this tutorial you will need to have the "layers" dialog box open. If you do not, press ctrl + L.

I will use an iPhone App Style logo in this tutorial.

Then duplicate the layer as so:

You should now have this:

Then press ctrl + a + x + v and use the elipse tool to draw an oval about half way up the logo. You should have something like this:

Then go fill it white and turn the opacity to 30%.

Then just press ctrl + m and you're done!