I use dkmatdrum's clock trick all the time. Also, I use either the setting sun or the north star to figure out where the sun will rise in the morning, so that I can set up my tent to get morning sun or morning shade, depending on the type of mood I'm in.
Another trick I often use is extending my arm and counting the number of hand widths between the sun and the horizon. Depending on the size of your hand, you can figure out how approximately how long till the sun sets. Each one of my hands is roughly 45 minutes till sunset. A very rough approximation indeed, but it gives me a general idea.