Pbartender said:
Another key, is to keep a small notepad handy and WRITE DOWN NOTES AS YOU MAKE STUFF UP. You must be constant, or the players will figure out your mistakes sooner or later.
Quoted again to emphasize the importance of this! I'm a pure wing-it DM. I almost never fully stat out creatures/characters/encounters. Instead I decide what encounters will "probably" happen, and put post it notes on the edges of the books I would be using.
For any NPCs that the party might encounter, I write down Level, HP, AC, Attack, DM, Saves. I dont even follow the rules for those stats either. I dont write down feats, spells, skills, etc. If an NPC needs to make a skill check, I just add 4 to the level of the NPC and assume that is the bonus. Divide by two or four if the NPC would not be expected to be skilled in that area.
First time the party runs into an NPC, I use whatever feats I find convenient at the time, and write them down. Next time the party encounters that NPC, they have the same feats.
Spells are the tricky thing. How do you wing a 10th level caster? they have TONS of spells to choose from. Again, the trick is to be consistent. If you want to wallop the party with a cloudkill, use It. Then take note that the NPC has cloudkill, so you dont acidentally give your sorceror too many spells known.
The pitfalls of dynamically choosing spells is that you will find yourself using the same spells again and again. So just before game day, pick 5 spells that the party has never seen before and try use them. Everyone expects Fireball, no one Expects a Sonic Boom.
Hope my players don't read that....