It's all about the chemistry between the RPers. That, more than anything else, is what you'll want to cultivate. Precipice of War is a sandbox NRP, and it's not only managed to last (in various forms) for a little over five years now, but we've always managed to work a story out of the chaos.

I think the trick is to get your RPers working together, and to shepherd the story not in a pre-decided direction, but in whatever direction it seems to be headed naturally. Let the RPers come up with the story, and work with them to connect the strings so the disparate pieces get sewn into one whole.