The campaign that I currently run has about three over-arching plots going currently. The characters in the campaign may not directly participate or influence each of the plots yet, but they are developing around them, and at some point in the campaign, each plot will affect the PC's in some way, shape or form.
I typically run one of three types of adventures:
1). An 'arc-episode', where one of the three major plots is advanced through the player's interaction.
2). Character background story, where one of the elements of the PC's backstory is addressed. The backstory item may be resolved, or it may become more complicated, leading to further Type 2 adventures down the road.
3). The one-off adventure, normally a quickie, one-nighter thing that has nothing to do with either 1 or 2 above, just a quick quest for the players to fulfill. A lot of times this will be a converted 1st edition module like The Sinister Secret of Saltmarsh, or The Lost Caverns of Tsojcanth.
Mixing and matching this combination of adventure types keeps the players from growing bored with the same theme over and over, but brings them back to the main plots just often enough to keep their interests piqued.