Sadly theres not much that you can do to prevent paint fading, especially in sunnier climates. The key is to make sure the prep is as good as you can afford and, some paints have better resistance than others but it depends on the colour you need. Then its just a case of monitoring and cleaning occasionally, and where any paint starts to flake get it re-treated and touched up before any corrosion can set in, the priority is to keep the airframe safe from corrosion. Re-painting is cosmetic and can be done when time/resources/MONEY allow.