I found that whenever I am in a sorta scary looking place (or in pitch black) my evocations work best.
For example, my garage is a pretty scary place. I hate going in there at night. Two people were allegedly shot there, so I've always avoided it.
I tried an evocation the other night in my garage with all the lights off. I was hyper-aware of my surroundings. I found that this made me much more sensitive to the spirit's presence. It was a very 'charged' experience.
I figure, when the you are not really using your eyes all your other senses will go up. On top of that, it is remarkably easier to see with your scrying eye in the dark.
I highly recommend it.