This may sound like a lot of work, but it's pretty simple, really: I have a ton of Dungeon magazines, y'see. So the other day I downloaded an Excel spreadsheet index of the past four years of Dungeon from 3d6.org, sorted them by level, and picked my favorites that were roughly the right difficulty level for the party. The next step is to go through them all and make notes about how they can be tied together and into the campaign background. One of them works wonderfully as a bit of background for Diglett Pindernipper, for instance, while a different one ties in very well with the Guild of the Arcane Path. Got a "kidnapped daughter" in Scenario A? Make her the "mayor's daughter" from Scenario B! Got a "devious assassin" in Scenario B? Have him be the "shady smuggler" in Scenario C ... and so on.
The neat thing about this is that I can use the index as a roadmap for the whole campaign. I did a quick-and-dirty outline of the game by the party's level, breaking it into chunks. For instance, the Vanity - Totentanz - Descent Into Madness arc was the "4th-6th level" chunk. By making a list of appropriate scenarios and key events, I now have the basic plot for the 7th-10th chunk, the 11th-13th chunk, the 14th-17th chunk, and the 18th-20th chunk. It's still very flexible of course, because the players may very well go off in directions that I haven't anticipated. ("Who wants to fend off necromancers? Bissel's a rotten little country anyway. I want to go find the Ghost Tower of Inverness!") But knowing that the characters are likely to someday find themselves in the "Lost Temple of (SPOILER)," even if it's not 'til they're 14th level, I can occasionally drop tidbits that point the way now.
The best part of this whole thing, is that having this "roadmap" makes me a lot more enthusiastic about my game, and a lot more interested in sticking with it to a satisfactory conclusion. I want to see how Jaer will handle (SPOILER) when they're 8th level ... I want to see them go to Verdhaven's monastery at 14th ... I'm curious how Kyriela will react to (SPOILER) at 16th. Now, instead of a vague "I want the game to be cool" attitude, I can look at my plan and say, "This piece will be cool because of X. That piece will be cool because of Y." It's a remarkable shift.
I have high hopes for this new approach. :) I'll keep you posted as I go!
 BTW, NWN 2 coming summer 2006! W00t. :)