The best groups I've played with or DM'd have not just been the games that were run the best, or offered the best adventures or settings, they were the games wherein the players were just as interested in developing the game as the DM. . . . It seems like having a way to briefly describe the players' role in the development of the best campaigns would be a good thing - just as important as providing a good setting, good adventures, and the best suited ruleset.
My best experiences with roleplaying games include players contributing to the development of the campaign as well, and thinking back on those campaigns, here's what stands out for me.
Setting and managing expectations: Proactive players rarely just happen. Many players expect a campaign to be prix fixe, and if you're serving à la carte, it's important to put it in their heads from the giddyup so they don't go hungry. Players should be encouraged to see themselves as more than mere consumers of a campaign, and in my experience this begins before character generation takes place.
Harness the rules: Some games make this very explicit - World Burning in Burning Empires comes to mind. The players are active participants in the process of creating the setting and the conflicts which arise from it, so at least some measure of campaign development is hard-coded into the rules. More traditional roleplaying games proceed from the assumption that the referee is responsible for creating the setting in which the game takes place and the conflicts which drive the action; in some systems, some player development of the campaign may take place through their characters, often through rules like GURPS' Advantages and Disadvantages or even through the 'stronghold-building' aspect of name-level 1e AD&D characters
In Flashing Blades, characters have Advantages and Secrets, some of which introduce conflict through established relationships, such as Sworn Enemy, Secret Loyalty, or Favor, while others create connections to institutions in the game-world, such as Member of an Order.
I should note that while I believe using the rules to establish initial connections to the setting may help, they are not a substitute for developing those connections through actual play.
Reinforce the genre: The game genre offers clues to what characters in the campaign are expected to do. For example, cape-and-sword suggests roguish characters who spend time carousing, wenching, gambling, and dueling. It also includes romance, intrigue, a struggle for honor, and advancing one's station in society.
In my experience, the memes conveyed by genre encourage the players' freedom to improvise by handing them a lever with which to move the setting in actual play.
Dangle carrots: The game's rewards system drives player choices.
Let me say that again.
The game's rewards system drives player choices. If a player wants her character to get better at swinging a sword, and the way to get better at swinging a sword is to engage in duels, then the player is more likely to pick fights.
If the game rewards in-character behaviors which serve to develop the campaign, then the players are more likely to pursue them. In Flashing Blades, for example, the career rules offer the adventurers the chance to improve their Social Rank, whereby they gain access to wealth and influence. As the campaign progresses, increased Social Ranks puts them in a position to exert meaningful changes to the setting in actual play.
Provide a dynamic world: In my experience, the more the game-world feels like a living place, the more the players and their characters will feel like they can effect change. The classic example of this is 'clearing the dungeon' - if the adventurers return to town after defeating the cultists in the ruined moat house and discover that there's no room at the inn because merchants are passing through again, and there's new construction to accomodate families moving to the area thanks to its safe reputation, then the players will feel the effects of change even if they didn't set out to cause those changes directly.
So that's how I do that.