Wow, there's been some great advice in this thread. I think we're getting better at this, gang!
Just to chime in on my favourite little tidbit that I haven't seen posted yet:
Active NPCs
You don't need a "plot" per se, if your NPCs (good guys, bad guys and those in between) are always running around and causing trouble. I have a entire folder of little text files, one for each of my key NPCs or NPC groups, and every time I have an idea as to what one of them might be up to, I note it down. I run through those files once a week or so (before doing much planning for any given session) and make sure that all my NPCs are off pursuing whatever their goals might be, and looking for ways to tie that into what the party is doing. So if the expert spy has just gotten double-crossed by her employers (who happen to be the party's key nemesis), perhaps she'll seek out the party and offer to help them out. Or maybe she'll go digging for dirt on the party that she can use to get back into her former employer's good graces. And once that dirt comes up, you know it's not getting put back anytime soon...
The point is, if you've got a fair number of NPCs who are actively pursuing their own goals, and said goals intersect with the party, you don't need a "plot" for your campaign. Your players will have to either start taking action, or action will start being taken against them!
My players have learned to pay attention to what's going on in the background, because they realise that it's going to affect them somehow.
In the very very very very very excellent chapter on running adventures in the d20
Call of Cthulhu book, Monte Cook says that good adventures aren't stories in and of themselves -- they're the
PROMISE of a story. The story is what happens when the players storm into the DM's encounters. The description of the encounters, what the DM has in his notes beforehand, is just the promise of the story.
No guarantees that the story you promise is the story you get, though.