Two of the hardest things for an RPG campaign are the beginning and the ending. Like a popular television show that has lost its spark, a campaign will sometimes just limp along lifelessly until people just stop showing up and it dies a lingering death. Letting go of a campaign is a hard thing, but sometimes...just sometimes, it has to be done.
One thing that I have come to terms with as a GM in the last few years is the idea that the open ended, potentially endless role-playing campaign isn't something that I am much interested in being involved with any more. I like the idea of a campaign that has a beginning, middle and endpoint to it. I think that it can make for more interesting campaign stories and character development. I know that this isn't the standard for how a lot of people campaign, but I think it is one of those things that puts a bar to the entry into the hobby. Meeting 5-8 hours weekly for an indefinite amount of time just isn't feasible for many people these days.
I like the idea of the trilogy as the model for a roleplaying campaign. This allows you plenty of space for the creation of worlds and characters, without the time sink of an "endless" campaign. With the idea of the trilogy as the model, that gives you plenty of breathing room to write your worlds large. Come up with the idea of the overarching antagonists of your campaign. Flesh out the idea that the campaign is leading to an ultimate confrontation with them. Have agents of the antagonists draw the characters into the upcoming conflict, through guile or outright physical confrontation, and use this to have the characters discover the plots of the antagonists. Draw the characters into confrontation with the antagonists and they will want to thwart their plans. Lead to final confrontation and close curtain.
Of course the campaign doesn't have to stop there. You can build a new trilogy out of the changes wrought to the world in the aftermath of the final confrontation. This could be a way to make something that is akin to an "endless" campaign, but with the option of periodic jumping off points.
You can come up with a loose idea of how the campaign will progress, something as simple as "the characters should be in conflict with the big bad within 8-10 sessions," or as complicated as you want to make it. I prefer the idea of a looser approach because 1) you don't want to put your campaign on tracks and 2) no plot survives the introduction of the characters. You want enough flexibility to be able to move the campaign with the actions of the characters, and you want to be able to integrate the ideas of the players into things. Players like it when their hunches about NPCs or a campaign comes true, even if it is because you think that their weird ravings are better than the ideas that you have come up with already. It invests the players in the game and makes them excited as well. This is why it is a good idea to keep things loose enough that the campaign can alter course without it seeming to be artificial. Players aren't stupid, they notice these things.
Sacrifice is always a good way to end a campaign. No, not the kind that summons an Elder God (that is better as a campaign starter), but the type of heroic sacrifice where the characters die...but they know that their deaths bring about a better world. It can be important to remember that being a hero isn't having a laundry list of powers and abilities, it means doing the right thing at the right time...even if that brings personal harm to you. Not everyone wants to kill their character, but having a campaign with a beginning, middle and end to it can encourage this kind of thinking in players. Sometimes you just don't have any other choices when your back is against the wall.
If you do use the series of trilogies method you can still do this. In the source material for many fantasy and science fictional genres, it isn't unusual to have a generational aspect to the stories. Did one of the big heroes give her life to close the demon portal and save the world? Perhaps her daughter must deal with having to live up to that legacy while wanting to create her own in the world. The idea is to not think of these things as limitations but as opportunities for new stories and new chances for roleplaying.
With a series of trilogies you can also explore shared GMing, with each trilogy rotating the GM chair to a different person at the table. This gives everyone the chance to either GM or be a player, and it bring different perspectives to even the same world as each GM will bring a different perspective to things.
Try something new and different with your campaigns. It may freshen things up and bring new life to old games. It will also help out with the campaigns that slowly atrophy and wither away.
