A lot of it comes down to the consistency of the playing group. You need to meet often enough to maintain the continuity of and interest in the campaign. You also need the majority of the players to attend every session, for much the same reason. Finally, the players have to be interested enough to remain involved in the campaign, rather than merely attending the game sessions (basically, that's the difference between a player who needs an extensive recounting of the game's events at the start of every session, and a player who just needs a few details clarified).
If the players can't commit to any of the above, you might want to go for something less ambitious (even a standard-length campaign). You'll probably enjoy the game more - long campaigns aren't necessarily for everyone, or inherently better than any alternative.
There are a couple more things DMs can do to facilitate a long campaign:
NPCs the players love, or love to hate. Generally, it's the roleplaying aspects of a campaign that will sustain it, rather than just the system.
Ongoing plots. If a campaign is equivalent to a season of a TV show, then a good campaign is equivalent to a show with a solid season-long plot (Angel rather than the A-team, for example). A very long campaign might rival Babylon 5 in scope, and needs a similarly large plot. How close to the foreground the plot is is, of course, a matter of taste.
Forward planning. This allows you to drop in hints of upcoming threats ahead of time, and allows players to plan their characters similarly. It also feeds into my next point:
Easter Eggs: Players like it when the pieces of a long-running plot fall into place. If you give out the clues to a puzzle over months of a campaign, the group will have a real sense of accomplishment when the whole thing is revealed. It takes adeft hand to pull this off, but if it works, it can be spectacular.
Example: I once ran a five-year (real time) Vampire campaign where a single plot line appeared off and on throughout the campaign. Eventually, events from the very first session finally came together into a wonderful session of in-character play five years down the line. No dice were rolled that night, but it remains one of the key sessions in the course of that campaign.
One last thing: know when to quit. The campaign I mentioned above ran out of steam after about three years. We should have wrapped it up sooner, but made the mistake of going on long after the magic faded. Apart from a few great moments, we would have been better playing something else.