Mmmmmm ... meta plot. My favorite part of GMing. I've always struggled with the small details of adventure creation, but meta plot is where my heart is at.
A good meta plot has a mixture of structure and malleability. Structure and planning are needed give the sense of events moving outside of the players' control, for consistency, and so that details can be foreshadowed and revealed slowly. Malleability is needed because the plot still needs to be able to bend and sway with the players' actions and goals, and build towards an epic, meaningful conclusion without railroading the players into a certain course of action. "An ancient organization is working towards awakening an ancient evil being that would threaten the world as we know it" is a good starting point for a meta plot because it describes what's happening in the world, without dictating the players' actions with regard to the plot.
A good meta plot is overarching but not all-encompassing. It should help tie the scenarios in your campaign together, but shouldn't be so omnipresent that every adventure revolves around the meta plot (at least not in ways that are immediately obvious). Ideally, references to the meta plot should start slowly and then build and intensify as the campaign continues. TV shows do this all the time, and actually, the Dresden Files books are a great example of this type of storytelling technique. The first few novels are small-scale, more noir-ish and self-contained, while later novels get steadily larger in scope, and begin to introduce elements that tie the books together, while still being able to stand alone as novels in themselves.
I could go on forever, but those are the two most important guidelines I try to remember when creating meta plots. If you're looking for some specific ideas I'd be more than happy to help with those as well.