Sit down Pardner, throw a log in the fire.
I have always gone for continuous camapigns where the players complete various adventures as part of an on going saga. One of the best ways to keep this fresh is to make the posee members really important to the plot of each mini adventure, more importantly, give everyone the spotlight.
You will notice that the smallest of deeds "like biting in half a prairie tic" go down in the posses memories, the posee comes together and creates te atmosphere for you. Leaving you free to take the drivers seat.
Serveral atmospheric touches I can share with you are:-
If you trap the posee in the middle of the arizona desert, tell all the players that they can't actually have a drink until their characters find something, then just leave 1 bottle of something in plain view and leave the room periodically. They will scramble for it. Obviously let them get away with it.
Accents and impressioins really do make a difference, try and develop a variation on yer speech.
Don't be afraid of detail, give em just enough so that their mind does the rest.
If all else fails, grab a plot line from a completely non related film, one of my most popular and grusom was using the bugs out of star trek 2 wrath of khan.
If things really go stale......kill somebody....a character.
There is probably a lot more I could say but at this time in the morning my head does not want to work, ill probably come back to this one.
regards and all that
Peter King
AKA The Judge David Gabriel & Chance Black