- Player buy-in
- Allowing players to mold the scenes (and scenery)
- Player control over certain NPCs
- Starting all of the above as early as possible
I am currently running a game of Unknown Ponies: Failure is Awesome (a My Little Pony: Friendship is Magic spinoff game of Unknown Armies), and I've set the campaign far enough in the future from the show's canon that all of the characters with a normal life-span are dead. (Thus, the four Princesses and Spike remain, as do Celestia's and Spike's pet phoenixes.) This was explicitly designed to allow everyone to swing their mighty Hammers of Headcanon and Flails of Fanon, as well as generate our own content.
While I laid out a decent amount of structure for the campaign's lore and setting, I didn't create everything -- there's no way the average player would read everything written about the setting if "everything" was too detailed. Whenever one of my players would ask me a question about the lore, I would either make something up on the spot which fit into my own setting's canon, or reply with something to the effect of, "What do you think the answer should be?" In the latter case, I would incorporate their response into the setting for the game.
This gave my players a great deal of power over the setting early on, increasing their buy-in. The actual gameplay was similar; I would avoid describing a scene in too much detail until asked. I would either make up a response or description on the spot, or let the player have his or her way with the scene. Occasionally a skill check would be involved (roll Notice... sure, some of these rocks are made of phosphorous, why not?) but often the scene was built as much by my players as it was by me.
I think the final key to what I've achieved in this game was giving my players control over the NPCs which represent their immediate family. Not only does it lift some of the burden off of me (and I don't feel like I'm a crazy guy talking to his sock puppet), not only does it let the players have their family members act in accordance with their character's concept, it multiplies the ways in which the players can present those new plot hooks.
At the end of the first story arc, the players accidentally got a mountain blown up, creating a large number of refugees. At the beginning of the following session, I told my players that there was a few weeks of downtime, and I asked them what their characters had been up to in the duration. One of the players said he'd been helping his father at the docks with supplying the relief efforts.
(Note: The PCs are all underaged "blank flanks," think ~10yr human equivalent)
News that the player was working the docks then reached the school principal, who called in Foal Protection Services, which launched into a story arc centered on exonerating the player's father. While the player may not have intended for such a story to occur, it nonetheless occurred entirely due to player action (deciding that he'd been working the docs to help with the relief efforts, and letting that fact slip and reach the principal's ears).
The current story arc has thus far involved a mysterious meteorite that landed on the farm owned by the family of one of the players. The players got Princess Twilight to come help investigate (through a relationship they forged in the first story arc). During the investigation, the father of one of the players revealed to Twilight that he and his wife had discovered four of the necklaces which had once held the Elements of Harmony -- powerful artifacts, indeed -- and were holding them secure in their basement.
I had had no intention of involving the Elements in this campaign prior to this revelation, but I rolled with it. Twilight retained control of her tiara which has once housed the Element of Magic, but the player's father revealed that two of the necklaces they found had been moving under their own power, somehow. With Twilight's assistance, an area was found where the final missing necklace should be, and the player's father declared a desire for the PCs to go looking for it.
The PCs being children and her friends, Twilight was against the idea of letting the go into the desert, near the border of the Badlands; an argument sprung up between the father and the Princess. Eventually, however, Twilight conceded to the adventure on the condition of parental chauffeurs and the assistance of Spike the Dragon.