Most GMs have at least one story arc in mind when they start
their games. This usually leads to the common isue of being completely
unprepared when that story line is concluded... and often this is further
complicated by the PCs not having a good reason to stay together any
more.
You can sidestep this issue if you have more than one storyline in
mind. Generally one huge, generalized, overarching theme (such as "will
magic or tech become central to this world?" or "Will Dr. Monstrous
succeed in taking over the world?") is useful in giving motivations to
NPCs. That, coupled with PC motivations, usually helps in generating
more short-term story arcs that will keep your players interested and
moving forward in the game.
Examples of potential GM thoughts:
"Okay, as far as personal missions that the other team members
are helping on...
"The rogue has several love interests in the city, and one of them
is Dr. Monstrous' son. The martial artist is searching for the secret
ninja master to learn more about the evil doctor's plans, and the gun
bunny is busy trying to find money for the mortgage on his mother's
house... which has nothing to do with Dr. Monstrous, but we won't tell
his player that.
"Now... how can I move along the over-arching story line of
Dr. Monstrous' plot to take over the world, as the PCs help each other
to finish their individual quests?"
If you're really stuck, you can always try using a random selection on
the 36
Plots page. This is an off-site page, however, so clicking it will
open a new window for you.