Yesterday I went to the hardware store and bought a pair of those outdoor solar powered lights that you are supposed to stick in your lawn (they come with a stake). During the day they charge two double-a batteries, then two small (but bright) LEDs automatically come on at night, and they also have an on/off switch.
I took one apart, and am doing the following:
Taking the top, circular disc that has the solar panel + light detector (to know when it's dark) and placing it on the roof of the coop.
Extending the wiring so that it wraps under the eve of the coop roof and through a ventilation hole, into the coop.
I removed most of the plastic from the internal apparatus so the only thing that goes inside the coop (to be screwed into a beam on the ceiling) is the LED lights/battery pack.
I also removed the switch on the back, and wired it so it's just outside the coop.
My hope is that this will be enough lighting for the chickens to find their way into the coop at night, and the only thing I'll have to do is come out and shut the coop door and flip the lights off. Right now I have to come out and physically put them into the coop, so it will be an improvement (if it works).