Keep in mind, I'm using GIFs to save space, the quality of glassiness is better if you use JPG or BMP or PNG or whatever.
Anyhoo....
First start off with your sig/pic/whatever.
Here's a sample sig I whiped up in like 5 minutes.

Now create a new layer and make a selection like so:

Get out the gradient tool and set it to this gradient:

Now from the bottom of the selection on your new layer drag upwards and you get a weird gradientish thing. Set it to 70% opacity. You should get this:

You could stop here if you want or you can continue on to the next few steps.
Get the Eliptical Selection tool out again and make a selection like so:

Now hit backspace and you should get this:

You can also use less of a selection space for the gradient part, whatever you like. You can also use different opacity setting but I think 70% looks the best.
Well, now you're done. Enjoy and remember to save as a JPG or PNG.