Here's some advice about how to effectively bring a long-running campaign to a close. I've done this a few times, most recently in the several campaigns I was running before moving from Chicago to Seattle. So I've put down several points of advice for GMs doing the same. I also want to give a shout-out to Mark Seifter, whose excellent thoughts about preparing a final encounter will appear in the upcoming Pathfinder #144: Midwives to Death.

* Get the Gang Together. When you're bringing a long-running campaign to a close, you should plan for everyone to be there for the last session or two. If that means adjusting schedules or obtaining babysitters more than normal, it's worth it;  emphasize this is the last hurrah and as many players as possible should make it. These are more like special events than the usual gaming groups have been, and the players should all be aware of it.

* Spend Extra Time Recapping. Although I've mentioned before how every session ought to begin with a focused recap, now is the time for the most important--and maybe lengthiest--recap. You're telling the heroes' stories as an overall narrative, leaving out only the conclusion that is about to play out. The players should all be clearly aware of what they've done before and what's to come next. This is also a good time for some pat-on-the-back emphasis as well; when talking about the victories the heroes have made in the past, be congratulatory about them. You also should absolutely be spending this recap time foreshadowing the conflicts to come in the upcoming conclusion--preferably by pointing out things from the past that are directly relevant ("and as you struck down the Demon King, he muttered something about the Time of Burning Souls being at hand, although you haven't yet learned what he meant by that.") 

* Time for Nothing New. The final session(s) shouldn't introduce new characters, threats, or other plot points. You're wrapping things up, not teasing new events. If you need an NPC, pick one from the campaign's past; even if it's tenuous ("the merchant you meet is the same mustachioed vendor that gave you all the job outside of Fallowvale"). Minions or other monsters should be ones the heroes have face before, or powered-up versions of ones the heroes have faced before. This all creates consistency, and this is your last opportunity to create campaign consistency. Think of the campaign conclusion as a "Greatest Hits" that works with existing material rather than creating a whole new album.

* Call-Back Like Crazy. You should take every possible opportunity to remind the players about neat things in the campaign's past. Have enemies flee from the heroes because they're the ones that killed the such-and-such monster two levels ago. Bring the fairy swarm that bedeviled the characters last year--a foe they loved to hate--back into the game for a scene. If you can make a troublesome past creature work into the plot to enhance the present impact, so much the better; if the heroes couldn't ever disperse the capricious fairy swarm, for example, perhaps the campaign villain's Necromantic Miasma kills off all the fairies at once in a tragic way. This call-back advice also goes for out-of-game things. I've found GMs generally try to be serious while players are cracking jokes. Now's the time to get in on the player jokes, too. If your players insisted on calling the pit fiend Liebdaga "Lady Gaga" the whole time (yes, true story), perhaps an NPC "accidentally" calls it Lady Gaga, too. If the players have a favorite local restaurant to get dinner from or favorite cookies to munch on, splurge for those. 

* Concoct Situations for Just That One Thing. Many players hold on to one-use items for far longer than they really need to; now is the time to create situations that need that elemental gem or bead of force their characters have been holding onto for so long. These don't need to be the normal uses for the items, either; if you have a diabolical machine that's cooled down with a water elemental gem or a smoke monster that can only be imprisoned by a force effect from a bead of force, that's good. The more you can lead players to think, "oh, I'm really glad I held onto that!" the more satisfying they'll find the finale.

* Have Something for (Most) Everyone. Strive to have a special scene, memorable victory, or other opportunity for each player to shine. If the monk finally avenges her murdered mentor or a witch irrevocably bests the hag coven that's been trying to tempt her, that lets those specific characters wrap up their own arcs. Think about each character individually, and not just the party as a whole, in deciding what threads to wrap up. Note that you don't need to indulge absolutely every character's story, though; if the previous adventure was all about the fighter getting her family farm back, her individual story already reached a satisfying arc and you don't necessarily need to rehash it. If the bard has defined herself by always seeking her missing teacher (in a way that hasn't been a focus of the campaign), it's okay to send him off past the campaign conclusion still looking.

* Answer As Many Lingering Questions As You Can. You simply won't be running this campaign any longer, so there's no reason to continue to protect your carefully hoarded secrets. Now's the time to point out--in game--relevant secrets that the heroes never uncovered but which might still be bothering them. A sage opines that the Bone Priest kept coming back because he was a rare type of lich, or the new baron suspects that the ghost of Baron Umphel couldn't rest because a book (which a player took) remains missing from his beloved library. There isn't going to be any way you can answer all the questions in-game, and that's just fine; not every mystery absolutely needs an answer. Be sure to focus not the most relevant ones, and the ones the players have found most interesting.

* Future Game Balance Doesn't Matter. You're ending the campaign, so you should be far more liberal than you otherwise would be about treasure or other balance issues. The paladin finally receives a holy avenger or the rogue manages to steal a 100,000 gold piece diamond. A druid benefactor enacts a ritual to give every character a +6 natural armor bonus. The players should be able to enjoy these things for a little bit without having the campaign go off the rails, as this is their last hurrah. These dramatic rewards also make the players feel like things are reaching their most dramatic conclusion.

* Make the Last Fight Memorable. Although raw power is expected in a campaign villain, you should consider an enemy that fights in stages--you have to overwhelm his Machine of Burning Souls with positive energy to strip him of his defenses, or maybe when defeated the villain morphs into some other threat. The key is to have the benchmarks be based on player successes: they complete the ritual successfully, THEN they can defeat the Soul Guardian, THEN they can take on the demon prince directly. Each success opens up a new stage of the fight, which makes the battle both satisfyingly lengthy and makes the players excited about what they're accomplishing. 

* Consider a Post-Campaign Wrap-Up. Sometime after the final conclusion--maybe not the very minute you say "The End," but shortly thereafter, feel free to have an out-of-game chat session about the campaign. Mention things that really surprised you, mechanically or otherwise ("The Bone Priest didn't have a particular weakness to your spells; he could only fail that save on a 1 or a 2, and I just never rolled above a 2" or "I didn't think you'd actually steal from the ghost's library, but I just had to roll with that.") Here's where you can also answer any mysteries that you didn't get to answer in game--there's always a few of those ("yeah, that skeevy merchant was actually the corpse you found in his wagon. He'd been replaced by a doppleganger, but you folks jumped right to him being the Fallowvale Slasher instead. Justice served--he was a killer after all--but not in the way I'd planned.") 

Best of luck with enjoyable campaign conclusions!