1. Be an evil DM. Always make pc:s and npc:s suffer. It is not quite sane to be an adventurer and kill monsters all the time, now is it? If you don't make up new evil come over pc:s all the time, players get bored. Don't let this happen. Also, make certain, that every character who tries to 'settle for normal life' has nasty future ahead of them. Party in-fighting can be easily made part of game enjoyment, players all around do seem to enjoy it a lot.
2. Evil demons and vampires should always be willing to talk, if for nothing else, just to throw around comments how evil they are, and how they succeed in destroying the world soon, and none can stop them.
3. There are wizards out there, who actually get kicks out of casting spells. This should explain all those madmen, who don't seem to care whatever their own party is on way, when they feel like casting their favourite spell.
4. Some players always have their pc:s be orphans. Don't let them. Family plays very important part, even in D&D.
5. Feel free to give pc:s and npc:s vague abilities, you can freely remove, when they are not conviniant anymore.
6. Remeber, that 'boss' enemies can take much more damage, than basic goons. You can always add chacter levels too.
7. Demons have much more easier access to game world, than good deities would like their worshippers to think.
8. Use background music.
9. There is no such thing as unproblematic relationship. Not even one fighter has with his magic sword.
10. Everything you need to know about stopping evil thing, that is threatening to destroy *insert wanted object here*, is written down.. somewhere.