This tutorial is going to give you a little tip on making long rounded edge shapes like buttons etc. look more 3D.
For an example I'm going to use this button we made in the 'Trendy Button' tutorial:

If we haven't already got the rulers open then do so now (Control + R).
Now we need to use the guidelines do this simple clicking on the rulers and dragging the guidelines into place.
Now we want to place our guidelines like I have done below:

Be sure to get them perfect (I suggest using zoom throughout this tutorial).

Now we are going to use the Elliptical Marquee Tool with this button selected: and now make selections exactly as I have done below, zoom in and use the guidelines for help and holding shift create perfect equal circle selections on each side.
We should have something like this now:

Now using the Rectangular Marquee tool with selected still, connect the two selected circular sections together.
Should now have some like this:

Make a new layer (Layer> New> Layer...) and we're now going to use the gradient tool and using the colour white into transparent as our gradient ().
Start at the top and finish right next to the bottom of the selected section of our shape.
Exactly like I have done below:

We should now have something like this:

Remove the guidelines and now deselect the are (Control D), and because the edges are rather sharp use a Gaussian blur (Filter> Blur> Gaussian Blur...).
Set it to about a radius of 1.0 pixels.
We now have something like this:

We're now going to simple set the Opacity to: 60%.
WE'RE FINISHED!

We should have now created something like this: