If you want to increase the optical quality of the glass, also do the insides of the windows. I use Rain X (in the black box, or packet if you can't buy boxed). Apply on a cloth, and once it's dried, use a proper microfibre cloth designed for cleaning glass to thoroughly rub the insides of the windows to ensure you've removed all the excess or it will appear smeary at night time. This stuff definitely helps to increase the optical quality of the inside, used alongside the Rain X Yellow on the exterior glass. Rain X yellow obviously once you've got the overspray removed, although this stuff will move a bit of it, if the Clay Bar has missed any bits.
http://www.rainx.com/Products/Windshiel ... i-Fog.aspx