There is no secret to longevity, it's just about having good players and a good GM running things.

• As already said, schedule your games at regular intervals. Weekly, every 2 weeks, once a month, it doesn't matter. Schedule them in advance and make sure your players can make it. Inconsistent players or drop ins and drop outs can kill a game pretty fast.

• Make sure you know what kind of game your players want. If your players are unhappy about something, see what you can do to fix it within reason. This doesn't mean giving the players whatever they want, but it does mean trying to let them play in the direction they prefer, if your game can support it.

• Plan an ending for your game. If you want to start a game that you hope will eventually come to a proper end, it's important to know what that end will be. Know what your villains want to achieve and what the different arcs will be, at least in vague outline.

Nothing will kill a lengthy game faster than showing the players that their investigations and speculations are meaningless because you've been making it up on the fly. If you want them to get engaged and stay engaged, you need to provide engaging content. This also helps avoid GM burnout, because you are looking ahead and have a lot of content you're eager to get to. It also makes it clear when the end is approaching, because the game, like a tv show, will be heading towards it.

Some games will just fizzle out, there's nothing you can do about that. When that happens, I suggest figuring out which of your players you want to continue with, and trying again. Having a good couple players can really help a game last.