NPCs

1. Thou shalt not make thine NPC the hero, in action or in story. Thou shalt leave that to thine players.
2. Thou shalt not spend an enternity on backgrounds of NPCs that are only around for ten minutes or so.
3. Thou shalt use major NPCs as a guide for players, rather than informing them of everything as the DM.
4. Thou shalt not make major enemy NPCs tired clichés.
5. See commandment #1.
6. Thou shalt not promise 10 commandments when you can only deliver 8, 3 of which are fillers.
7. Thou shalt never attempt to use Old English when... thou... ist... when you suck at it. (Ok, so that's more of a guideline for keeping yourself from looking like an idiot... which I have failed to do... *drops the attempted "accent"*)
8. Finally, if you enjoy being a player but are the current DM, don't be afraid to use a major NPC as your character, you can almost get the best of both sides this way (keep commandment #1 in mind while doing this, however)

(Extremely) bad humor aside, these give a good general idea for using NPCs. Minor NPCs are easy, they are store owners, people on the street, bartenders, etc.; and should be treated as such. Major NPCs, however, should be treated much the same as a PC, but with the above guidelines in mind.