15) Originality is over-rated. I could spend a half hour statting up some ogres, or I could just throw three ogres and two ogre barbarians right out of the MM at the players. There is nothing about an encounter with a couple of fiendish Huge spiders that lacks.
16) It's not hard to come up with important numbers. If the PCs get into a fight with a 7th level warrior you don't have stats for, you can assume Str 14 and Con 12, you know his BAB is +7, give him a masterwork weapon and weapon focus, add armor and bam, you have enough info to run the encounter. Crafty shopkeeper? A level 4 expert can have a +7 diplomacy, assume Cha 12 or whatever, and he probably does not have Skill Focus, but he might have Negotiator if you think it's appropriate. Add max ranks in Appraise, max in whatever skill he uses to make a living, and add Skill Focus to that skill. Done.
17) It can turn into a bit of a joke, but there is a lot to be said for dungeons being exactly the same size as your eraseable battle mat.
18) If you use minis, try to base some encounters on what minis you have, rather than rooting around for minis for the encounter you would like to run.
19) When winging it, it's better for the encounter to be too easy than too hard.
20) When creating boss encounters, spend some time on them, but don't agonize over stats your players will never see. WRT D&D 3.5, just pick the feats you really want them to have, and use the rest for Weapon Focus, Toughness, Iron Will, etc.
21) The answer to "is there a __" in a town should usually be "Yes." Unless it seems unlikely, when it's "No," unless you feel inspired. Example: "Is there a temple in town where we can have Bobtomus raised?" "Yes." "Does this village of 2000 people have a merchant who sells magic weapons?" "No."
22) Most NPCs don't need stats, just names. If you keep a list, you can amaze and astonish your players with the vivid, detailed game world that exists mainly in their imagination. In reality, you've got a couple of towns and your scrawled list of NPC names.