As other have said - make sure the sunglasses are a neutral tint so as not to interfere with colour vision, and not polarised so that they don't block digital screens. Also don't get photochromatic as perspex screens block much of the UV light they need to activate, so they often don't go dark enough (this is aircraft dependant but you get the same problem in lots of cars).
Tell the optician why you are looking - they do so much mundane stuff that they often love doing something different!
Try to make sure that they don't interfere with periphery vision, either by having them sit very close to your eyes or have larger lenses. I have prescription aviators (can't remember if there is a brand on them). Because of the large lens surface area, the lenses have to be thicker than normal. My prescription isn't huge (around 2 - I'm short sighted) and its fine for me - but its worth thinking about.
I have a gradated tint on mine, which seems to work well.
Best of luck finding something that works!
Last edited by riverrock83; 8th Mar 2013 at 11:33.