A long term campaign is extremely hard to maintain. I have been a player in 3 aborted campaigns and found the experience so frustrating, I started up my own. The first thing you need is willing players, people that are willing to commit to a regular game (once a week, 2 weeks, month, whatever). Also, make sure that you don't have any conflicts in a meta game sense. A campaign is a large commitment and you're going to be spending a lot of time with these people. If you don't like a person, or their style of play, don't be in a campaign with them.
That said, I find it's best to establish the characters history first, before the campaign even starts. This may be a no-brainer, but I didn't really hold my players to this at first and it led to a lot of aimless drifting early on. Fun, violent, aimless drifting, but drifting none the less.
Having character backgrounds established will allow you to think of the campaign in terms of your characters. The bully that beat up your PCs rouge at the orphanage or the childhood sweetheart could be recurring characters, or even the culmination of the campaign. For example, The Princess Bride, Inegu Montoya (sp?)'s 6 Fingered Man. Character background will also to some extent allow you to anticipate what the PCs will do that can determine events much later in the campaign.
In terms of generating interest, a campaign is best served by an inciting incident. For example, my evil campaign began with the PCs robbing a busy hotel during a celebration. They sent the large dumb halforc NPC to create a distraction, and inadvertently set a large fire that destroyed much of the city. You need something grandiose, cinematic even. ie. the serene town of Faer has recently faced a string of assaults from local goblins. Instead of just starting the campaing with this knowledge, have them witness an assualt. Maybe their house is destroyed in the fray.
Make it real, make it personal.
A question. What level do you generally start campaigns at? I started the evil campaign at 4th level, mostly cause I wanted to give the characters a chance to be evil early on. I'm going to start the Shackled City AP at 1st. I've even been started at 5th level before. Generally, it's cool because it starts the PCs off with some power, but I feel it may detract if the characters don't have a solid background, after all the 1st 3 levels are now important to their characters. Anyone have any opinions on this?