I start by looking about three or four levels ahead, and choose published adventures that (1) I'll enjoy running, (2) I think my players would enjoy playing, (3) fit in with the overall path down which the campaign seems to be heading, and (4) are within two levels either way of what the party's APL will be.
Then I do an actual outline, starting broad by sketching out the ties that bind my chosen adventures together. If needed, I insert bridging adventures (usually short, two sessions or less).
I continually revise this outline as the campaign progresses.
As the group moves toward adventures, I flesh the outlines out. I've found that it really, really helps me to understand exactly how an adventure designer intends for players to put things together in order to move from encounter to encounter, so I'm always explicit about that in the more detailed outline. This is also the point at which I make any modifications to NPCs, traps, or (if needed) to the overall plot. (For example, if an adventure is written for 7th-level PCs, but my PCs are 9th, I may need to account for teleport, one way or another.)
So, in my current Eberron campaign, so far:
I. The Forgotten Forge
II. Shadows of the Last War
III. Murder in Oakbridge
IV. bridging adventure, introducing a new PC and a conflict with Daask
V. Whisper of the Vampire's Blade
VI. bridging adventure, exploring the conflict between Daask and the PCs
VII. Grasp of the Emerald Claw
VIII. bridging adventure, escalation of the conflict with Daask
IX. Chimes at Midnight
X. Shards of Eberron (Crypt of Crimson Stars)
XI. Shards of Eberron (Temple of the Scorpion God)
XII. Shards of Eberron (Pit of the Fire Lord, heavily modified)
XIII. Quoth the Raven
From here (APL is 11) I have one more published adventure planned, then it will largely be my own stuff until the campaign finale at APL 16 or so. I'm looking into modifying some published adventures, but it's not promising. There are too many directions the party can go. There are four major threads hanging (Viktor St. Demain, a white dragon out for revenge, Daask, and an invasion of giants brewing on Xendrik), and while I can guess which order they'll take them in (Viktor probably has to be first), I can't say for sure.