Well, here goes my contribution anyway of what I think helps a game to be successful (perhaps not what you'd expect):
1. Always have the munchies ready, starving players end up not thinking straight and do stupid things (like freeing demons
2. Never keep the players up for too long playing, unless every single one of them are keen - as soon as you get one player sleeping at 2 am - the excitement tends to break down.
3. No hack 'n slash. (I always tell my players to go play Diablo if they want that kind of thing).
4. Add optional mini quests (actually, this is an idea I should've used before, but I'm gonna try this weekend - instead of leading the players one-way all the time, offer 'em mini quests so they don't feel they're being dragged along one particular adventure).
5. Award individual XP for good role-playing, wise-cracking....
6. Punch the players hard on their fragile noses if they speak OOC too much.
7. Kick 'em in the nuts if they start becoming too annoying.
8. Ignore 6 & 7 if you're a pacifist DM (I'm not)
Well, anyone got any extra tips (be more specific if you want).
I particularly wanted to know how to DM high-level characters, because that situation is fast approaching & things become extra complicated (lotta magic, ugh