The formula for hummingbird nectar is one part of sugar added to four parts of boiling water. Don't use red food coloring, honey, or artificial sweeteners.
The National Audubon Society recommends hanging feeders in the shade to discourage fermentation and spoilage of the sugar solution. It also recommends changing the sugar water before it gets cloudy, or about twice a week in warm weather.
Follow their recommendations for cleaning the feeders by using a solution of one part white vinegar to four parts water about once a week. If the feeder becomes dirty, they suggest adding some grains of dry rice to the vinegar solution and shaking vigorously. The rice acts as an abrasive. Be sure to rinse the feeder well with warm water at least three times before refilling with sugar solution. Look for more information at the Audubon Society Web site: Hummingbird feeders
More information on feeding hummingbirds and other backyard birds can be found in this publication from Cornell University: What to Feed Birds