- Think of simple concept for your game. Then complicate it.
For example, my initial idea was "Let's do an Order of the Stick style game, where the characters know and make in-character references to the rules," which I complicated with, "and let's make it a homage to the Dungeons and Dragons cartoon!" Seizing on the trope of "gamers pulled into their fantasy world" as a way to justify this lame excuse for a comedy game, I failed to explain the idea sufficiently to my players, who nontheless agreed to go along with it. Mostly because they wanted to try out 4th ed.
- Start immediately, with maybe a page or two of notes to guide you.
Preperation is for wimps and people who want a consistent story arc. I scribbled down some ideas for stuff they could do, statted up one NPC and grabbed the Kobold Hall mini-adventure from the back of the DMG to throw them into. I've pretty much been making it up as I go since then.
- Abandon all your cool ideas if they require too much planning. Also if you keep forgetting about them.
I was going to run it as a series of "episodes" with them levelling up after each one, but realised that a) they'd be levelling up far too fast and b) there was no way in hell the players were going to be railroaded into doing what I had planned for them anyway. My pet NPC ("Dungeonmaster") also keeps failing to turn up to pester them because I keep bloody forgetting about him, and can never think of enough clever things for him to say anyway.
- Steal pre-made dungeons, adventures and encounters from books or the internet and drop them in where they make sense, regardless of how much you have to tweak them to make them level appropriate.
The reality of your made-up world is more important than game balance, and you can do just as good job as the professionals anyway.
- Change the metaphysics/background/plot/whatever as you go to fit in player choices and changes of character.
Just say yes to everything and make it work later. Player fun is more important than the reality of your made-up world.
- Tailor the magic items to particular characters in the party by predicting what they would like, find useful, or enjoy.
You'll get it wrong, but no matter how easy you make access to magic items in the game world, they'll never bother buying anything better, so you've got to try.
- Pick monsters and plan encounters to suit the strengths of just one character; put these all on a map so they know exactly where to go; make it a major theme, plan the next few levels around it and give them an artifact to motivate them to pursue this line of the story.
Of course, if, say, the cleric's player has to drop out of the game for personal reasons, then they might not be so good against undead, and you'll have to scrap all your plans and make up a side-quest to get them out of the area for the foreseeable future.
- Do about half an hour's game prep sometime before each session.
Who has time for more? The players will probably do something you didn't plan for anyway. And don't bother learning monster abilities before a fight. You'll pick it up as you go and never miss anything important that could change a whole combat.
- If they roll a really good skill check, let them trick the dragon out of its treasure. Just make sure that the next dragon they come across attacks them before they get to open their mouths.
- Let the PCs acquire assets (such as ships) all over the place. Give them command over a small army. Encourage them to start their own religion.
After all, what could possibly go wrong?
NB. I'm not saying I regret all of these. It's mostly been fun, and hopefully is still being fun for the players. I just keep having to change my plans and ambitions for the game as we go. And I still suck at combats, which makes them drag a bit. Maybe the next time I start a campaign, I'll be better prepared and get it right.