What is the best product to keep decoupage from fading outdoors?

You would need some kind of clear coat with UV protection.

Most varnishes do not have UV protection built in as that is not necessarily their function. Your best bet is to decoupage something that is pigment based and not dye based. Pigment based product do not fade but they can change/darken. But dye based products are very fugitive. You can test by putting some of your items to be decoupaged in the direct sun (keep a control piece) and leave them for several days and then check against the control piece to see what the color loss is. Good luck.

Krylon makes a clear UV-resistant spray : " Provides a permanent, protective coating which protects art, crafts, and valuables against harmful ultraviolet rays. Protects against fading, coating is moisture-resistant and smudge-proof." I saw it offered online at at Walmart for $5-$25. There is also a Mod Podge Outdoors is also a product available that is advertised as being " All-in-one glue, sealer and finish for decoupage. UV- and weather resistant formula ideal for deck, yard, patio, and poolside furniture and decor. Certified AP non-toxic. It sells for about $15 for 16ounces. I attach pics of both products. Good luck!