Breadcrumbs. I sympathize with you, having created a number of vast and intriguing storylines for games only to have maddening instances where the players aren't biting. Here are a handful of ideas:
1) Have you set the framework for the story in your mind? Because if you haven't, this may be a good time to evaluate where you want the game to go and where the players want the game to go. If you can't get those two to align, the whole game will be a frustrating experience for all of you.
2) If you've got the story ready, it's time to think about hooks. You really need to personalize it for your crew, but in general they've got to have something that helps them discover the main story. Rarely can you expect players to be self-motivated enough to try and catch the plot on their own; the only reason that works in novels is because the authors are responsible for both characters and plot, and would be insane to antagonize themselves by creating a rift between the two. An example from a game I'm currently running: the PCs are a group of bored teenagers amusing themselves by exploring a local abandoned mine. They're going to stumble across something that hints at a much larger world beyond their village, and when they emerge from the mine, they'll witness a hostile army conscripting people from the town. It would be unlikely that all five of them would agree to up and leave town before, but now they have reasons to do so.
3) If you want something more subtle than the world-turned-upside-down approach, consider small quests that gradually expose the characters to the plot. Video games are famous for this approach, but it will work for gaming groups also. This is also a good way to get them to know (and hate) the BBEG and his minions, particularly if their success on the side quests is directly hurt by the actions of the enemy.
One final thought: left to their own choices, the players will rarely end up wandering in the direction you've planned the game. It takes careful planning and continuous prodding to keep them going towards your goal. Above all, your pushing needs to flow naturally such that the characters (and players) are excited about exploring the opportunity; avoid forcing them to do something IC that wouldn't make sense given the information they (not you) know, because that's the essence of "railroading" that most gamers gripe about
The key is making it seem like they have choices while making the one you want them to pursue the most interesting and logical option.