My typical progression for a campaign may be little unusual. In the beginning, when the PCs are newly minted 1st level characters that have just been created by their players, I run adventures that don't tie too closely into any character's background.
Replacing characters is much easier to do when said character has no integral connection to the overall campaign. When it's established in the first session that these six noobs are destined to save the world, it's a little hard later on shoehorning in a new character--like the convoluted solution on Charmed when the "Power of Three" needed a new Third after one of the actresses was fired.
I try to avoid this by waiting for the players to see if they like their characters enough to keep them for the long haul, and for the characters to get a few levels under their belt and therefore become more sturdy.
Now the other guiding rule I follow when setting up a campaign is to "let the game talk to you". When I first start a new campaign, I have "notions" of where I would like it to go. But it's important to be flexible with those notions, and let them develop organically from early game sessions into ideas and roadmaps.
* If the PCs find a particular bad guy troublesome or aggravating, consider upgrading him to BBG status (even if you have to bring him back from the dead). I did this in my first game, when a kidnapper escaped the party, and one of the players frantically searched the area for any trail to follow.
* If you dangle a particularly juicy plot line that the PCs don't follow up on, consider dropping it in favor of something else. In another game I thought the group would research a rare and evil tainted item they had recently acquired. When they left it aside for a foray into a new dungeon, I simply dropped that idea (somewhat).
* If the party shows a preference for dungeon crawls, think about recasting your campaign in way that features more of what the party likes. As I mentioned above, when the group was more interested in a new dungeon rather than hours of library research, I ran a delve, and added some of the same story ideas into that dungeon (through journals and history/religion checks) that would have resulted from the tainted item they possessed.
Every campaign is different, just like every player and every character. You can learn a lot about your own game just by simply listening...