Choose the appropriate paint for the surface. In the majority of cases, this will be a 100 percent acrylic (water-based) paint. According to the Rohm and Haas Paint Quality Institute, oil-based paints are more likely to fade and deteriorate, especially in sunny exposures.
Expect to pay a lot more per gallon for very good exterior paint. When it comes to paint, you usually get what you pay for.
Ask a contractor or professional painter what he recommends, or visit a paint store instead of a hardware or "big box" home store and ask for recommendations. A reputable professional will know from experience which paints last very well.
Don't avoid smaller or regional brands of paint simply because they're not reviewed by consumer product rating organizations. They are often formulated for the weather conditions in your geographical area, and hold up very well--in many cases better than some national brands.
Ask at the paint store about pigments. Some pigments (primarily some blues, reds and yellows) will fade quickly in sunlight, even in high quality paint. Avoid very dark colors if you're in a hot sunny climate. They don't last as long as medium and lighter shades because they get much hotter in the sun.