1.) Pick the right time and use it. Stop advancing timelines. Figure out where the storylines of the setting were best and set the setting at that time. You can update the scenario for modern sensibilities, but don't make the Forgotten Realms mistake and keep moving further and further into a future that either eliminates favorite elements or bends over backwards awkwardly to keep them around - do what Eberron does and tell the best stories around a time where the campaign setting is designed to tell the best stories... The War of the Lance. The Death of Kalak. The Temple of Elemental Evil is founded.
2.) Don't skimp. A supported setting should have a core rulebook and then support that fleshes it out. I want all the iconic monsters, all the iconic NPCs, all the iconic magic items, all the iconic mechanics (Psionics, Krynn's Moons, Defiling, Epic Level Wizards in Greyhawk, etc...) If you're giving us a classic setting, it should feel familiar.
3.) Miniature Support. Make sure that you provide us with the minis that are iconic to the setting that we may not have - especially monsters. I can fake Kitiara with a lot of minis - but a Wyndlass is a creature I do not have and is iconic to me for Dragonlance games. As everyone has different memories of their Dragonlance adventures at younger ages, there are a lot of monsters in each of these settings that we have never seen.
4.) Edition Agnostic. Write the books in a way that the core of them are edition agnostic. Separate the crunch from the lore so that when 6E, 7E, D&X, 9E, etc... roll around, we can be using the same books and just using the crunch supplements that are released for the edition, allowing you to support all of these setting much faster in the future.
5.) To Thine Own Self Be True. Keep things that do not belong in the setting historically out of the core of the setting. Any DM can figure out how to put a gnome in Athas, a Lightfoot Halfling in Krynn, or an Artificer in Greyhawk ... but we don't need them shoved in where they do not belong.
6.) Respect. Don't disrespect the setting by trying to be funny. Using humor to 'update' the setting is making the same mistake we saw in Super Hero movies for so long - the people in charge thought they were ridiculous, so they demanded they be campy. These movies started to be good when they were allowed to be fantastical and have serious emotion. Gamma World in 4E is an example of this going wrong. They didn't respect what came before and it was not well embraced.