As was written above, putting works close to windows or existing overhead light fixtures makes for good locations. If there is track lighting nearby, itís easy and inexpensive to add lights. I just did this for a show at a restaurant. It cost about $20 per fixture, including the bulb, and the results are excellent. For places where track light wasnít an option, I got some clip-on light fixtures from the Home Despot for about $10 each. These come with a 20 watt clear bulb, connects to a wall outlet, and clips on the picture frame. For wider works, I use 2 of these.
There are a lot of really nice picture lights on the market but the cost for most of the better ones is in the $40 or > range.
You can tweak prints to be lighter when you know the ambient lighting is going to be less than ideal, but there may be a problem at some time if the image is relocated to a different (read that more illuminated) location.
Clip-on light fixtures are inexpensive at least and if not pretty, they bring light to nearly anywhere there is an electrical outlet nearby.