Calculating the effectiveness of sunlight reduction solutions
The best is to block the sun from the outside. Options there would be:


plant a tree (pro: green; con: might take a few decades to get full effect)
awnings (pro: do great at blocking summer sun; con: costs a bit, takes some maintenance, can block views)


As for the windows themselves, are they high-efficiency? Ideally you'd have triple-pane glass with low-e coatings. If these are old windows, it might be time to invest in replacing them. 

On the inside, blocking it with blinds is a good bet. A light, reflective or solar shade is a good bet.

Lots of commercial establishments will use shades such as these:

http://idahocanvas.com/roller_shades.htm

(Link is merely provided as an example product.)