Expect for the worst and plan for it.

Never assume people will stay, because there's not a single successful group RP that didn't lose players along the way. Always leave space, both in OOC administration and IC plot, for replacements. Some people may close applications at a threshold, but I leave it open even when the group is stuffed (new guys go into queue at that point), as that will keep a steady flow of replacements coming. 

Don't even assume yourself will be dedicated all the time. Like Ellri said, a good co-GM can back you up when writing gets difficult for you. Depending on how much you trust your co-GM, you may also wish to tell them future plans for the RP. When someone else has the roadmap, they can continue the game even if the original GM is absent. Long time RPs tend to become community efforts.

I'm against having pre-assigned roles or characters occupying non-replaceable roles. Just one of these pre-assigned or pivotal players leaving can cripple the entire story. Instead, start people off as somewhat generic and define them as they go along.

Most importantly, don't be afraid to make mistakes. Consulting your co-GM, players and even potential candidates (in interest check), to smooth out your plans from different perspectives. You may restart or reboot an unsuccessful RP, while doing so, look back to see what went right and wrong, and improve based on these lessons.