Once upon a time, I would write out the exact plots I had in mind well ahead of time, sometimes even several seasons ahead, but then I'd burn out before I could get going (the first time I got to play Spider-Man, I had him plotted out from Year One all the way to Year Ten, then stopped midway through Year Two).  I usually find that when I try and scheme things way in advance, my writing suffers because I get so hung up on the conclusion that everything leading up to it isn't given much thought.

Nowadays, I just try to think of a scenario, throw the characters into it, and see what happens.  This requires a lot more understanding of the characters' personalities and how they would react to a given situation, but it also requires less prep work.  It's messy and a lot of the time the continuity gets all screwy, but I think it has a better "feel" than whenever I would spend months and months setting up a payoff that everyone saw coming a mile away.