1) Simply try not to get it dirty.
2) When it is necessary to clean, follow leather care instructions - generally using a small amount of water (damp cloth), and perhaps a mild cleaner.
The aniline leather that Apple uses for its cases is on the more delicate end of the spectrum, which is why it patinas and shows signs of use. Water and sweat will darken it. The best way to care for it is to avoid soiling it, and needing to clean it. That's the nature of the material.
If you choose to cook it with UV light, don't be surprised if it fades, or dries out. Harsh chemicals will remove the dyes and oils used in the tanning process as well.
3) If that's not suitable, buy a fake leather case instead, or replace it as necessary.