While I've never had much of a problem with game stagnation, I have talked about it with other players and GMs. I hope to offer a few tips to help prevent your own game from going stale.
Game Stagnation- A slow down of general interest, drive and passion in a game. This can be the caused by a variety of events including (but not limited to): major story completion, lack of GM ideas, players tired of their own characters, lack of variety, redundant game types/styles, concurrent but non-associated off-the-shelf modules, etc, etc...
There are quite a few solutions to this problem, but I prefer to avoid it altogether as it can leave a sour taste in everyone's mouth. Here's a few tips to keep game stagnation from rearing it's ugly head at your table.
Run Away!!: As GMs, we put the players in dangerous situations all the time. Sometimes they aren't prepared and they run away. Enemies can do that too! Throughout any given campaign don't be afraid to let a few people escape. You don't need to think it out ahead of time, just let some guys run away and fill in the blanks later! This way, even if the "main story" is completed in your campaign, you have plenty of fuel to keep the game going.
- Maybe that escaped evil cleric was actually the Big Bad Guy's adviser. Maybe he was secretly raising his own undead horde behind his master's back. The confusing aftermath of the main story would be an excellent time to strike!
- Perhaps one of the generic guards or thugs encountered early fled in grief because the party killed his father right in front of him. Ashamed of his cowardice and seeking revenge, Deadguy Jr. seeks the council of a local witch who helps him make a revenge pact with some evil outsider.
- Towsfolk X remembers the adventurers from a previous game and seeks them out to rescue Towsfolk Y. In this case, keeping hold of game logs and notes can be very useful. Players love to feel connected to the world around them - "Hey remember when you beat up my cousin in that tavern brawl? Well he's gone missing!"
Player Background: Before you start a campaign ask your players to provide background stories for their characters. Let them go wild! Remember, you share the world with them, it's only fair that they get to create pieces of it too. These stories can lead to major campaigns in themselves! In our current game, Brad is trying to figure out why his father -a legendary monster hunter- has devolved into a slobbering drunk. Luci, an avid entomologist, is searching for the mythical "ruby eyed emerald grasshopper," and Blanklee is trying to save his soul from eternal damnation!
On a side note, you are still the GM. Feel free to fiddle with player backgrounds in order to keep them on their toes. At one point, our "amnesia afflicted" bard was searching for his criminally insane identical twin and our thief was in search of the man who murdered her husband. About halfway through the campaign, they realized they were looking for the same man! If that weren't enough, as the heroes finally found the nutcase, he released his secret weapon... the truth! He told the thief that he didn't murder her husband, his identical twin brother (the bard!) did the deed! That was some downright M. Night Shyamalan $#*! right there!
As an additional tool, you should consider using the "3x3x3 NPC" technique in your game. I hate to admit it, because I was only reading about it a week ago, but I forget where I saw this idea originally. At it's most basic, you ask your players to come up with brief descriptions for 3 allies, 3 acquaintances and 3 foes. This gives you ample fodder to throw into any game or even to base an adventure off of.
Keep it Fresh: Did you just play 3 knock-down, drag-out adventures in a row? Break the cycle with a mystery adventure, trap dungeon or all-roleplay adventure. Take the players out of their element - put them underwater or in the clouds. Have them run into old characters. The point is - don't be a one trick GM. I may prefer comedies but I like to watch action movies and documentaries too. If you were a movie studio, the players would be customers, and customers like a little variety every now and then.
There are plenty more ways to keep your games from stagnating. Hopefully these few methods here will be useful to a few folks out there? What do you do to keep things exciting and fresh?
Until next time, Happy Gaming.