Everything I Need to Know About Plot I Learned from Running RPGs
1. No outline survives contact with the characters.
2. You don't have to have every room mapped out if you keep a couple of generic encounters handy.
3. However, if something is on the map, you had better be prepared for when someone inevitably decides they need to go there.
4. Don't make the supporting cast more interesting than the heroes; they'll keep drawing your attention at the protagonists' expense. This is fair to no one.
5. The search for fortune alone is probably the least interesting thing for the heroes to be motivated by.
6. If you don't give them something to fight, solve, or negotiate with at all times, the characters will get bored and wander off on their own.
7. If you come to a place where nothing much is happening, you should sum it up briefly and move along to the next good part.
8. Someone will want more firepower unless you come up with a very good reason not to give it to them.
9. At least some of the characters will discover that it's to their advantage to act like thugs at least some of the time. This is more interesting if you allow there to be consequences for it.
10. Archetypes are best used as a starting point for creating individuals.
11. Someone will inevitably want to try something not covered by the rules. It's more fun if you don't always say "no."
12. When things threaten to get out of control, remember that you are in charge.
13. The people you are trying to entertain have read the rulebooks, and probably the adventure scenario too. You don't have to throw something unexpected at them, but if you don't, don't be upset if no one seems surprised.