Go read this and ignore everything said therein. That'll get you started on turning a fun social activity into a torturous nightmare for all involved.

How to ruin a roleplaying game as the GM

Don't do any work for your adventures. Show up with nothing but a vague idea of what you'd like to happen, or don't even bother with that. Randomly improvise stupid situations, inventing scene description and characterization off the top of your head. Don't bother with consistency or continuity in any of it. Keep track of everything in your head.

Don't give a shit about what happens in the session; you're doing those ungrateful players a favor just by being there, so they should be happy with whatever you give them. And they can shut up about how your NPCs all have the same mannerisms and speech patterns as you. You're a writer, not an actor.

Plot hammer relentlessly. If they don't like the direction the adventure takes, tough nuts. You're the GM and it's YOUR world, gawdammit! The adventure goes the way you say it goes, and if they don't like it, they can leave. Your word is law.

Don't bother fleshing out the environment or character backgrounds at all. Sets are only a location for killing NPCs, and those NPCs are only there to be killed. Character interaction is for boring wimp-ass roleplayers who don't know how to hack properly.

Hand out the treasure in great heaping gobs. Everybody knows that collecting goodies is the only point of RPGs, so go wild. Give that third-level fighter a keen vorpal scimitar +4, and the new mage looks like he could use a staff of the archmagi. If you're running a science fiction RPG, substitute incredibly powerful pieces of alien technology for the magical items. Don't forget the gold/dollars/credits; every character should have enough money to buy anything they need. What's the fun in solving problems without using all the latest technology, and why should they go into a dungeon without a small army to back them up?

Don't give out enough treasure. Players should be forced to battle for every scrap of bread their characters get, and hacking is much more fun and challenging when your only weapon is a rusty hoe.

Your NPCs know everything you know and should act accordingly. Every one of them can cast contact other plane and speak to the almighty creator of the universe at any time. No weapons are hidden, no conspiracies masked, no trickeries and double-crosses unspotted. They will always zig when the player zig and zag when he zags.

Reward legitimate criticisms with grudge monsters. No, use grudge monsters whenever a player steps one nanometer out of line. Hell, use them randomly! It's your universe and you can do as you please. Massive XP and level penalties are also a good way to keep things running the way you want.

How to ruin a roleplaying game as a player

Never miss an opportunity to critique the GM's style. Every GM loves to have his adventures nitpicked in meticulous detail. If he's going to write lame-ass crap like this, he really needs your help on the next one. Bitch and moan constantly about how unfair his decisions are whenever they don't go your way.

Read the GM-only material. Don't restrict yourself to stuff like the Monstrous Manual and the Game Master Guide, read his notes too! Go through his computer while he's out of the room and read the notes for his other adventures. While you're at it, you might as well read all the background material on the world. Gotta keep up to date on those player tacticals or you might lose the next session.

Everyone knows that the point of playing games is to WIN! More treasure, XP, and levels = More fun. If you can humiliate the GM and your fellow players in the process, so much the better. Minmax relentlessly and pick the race that gives you the biggest combat bonuses.

On the same note, everyone also knows that RPGs are all about how many kills you can rack up in a night. NPCs are only meant to be a source of XP. Be creative! The peasant farmers and the shopkeepers are worth a few XP, and every little bit counts in the relentless quest for levels.

Henchmen and hirelings are actually robots meant to carry your stuff and absorb damage. They don't need backgrounds or characterization, nor do they need to interact with anybody else. If you actually bother giving them a share of the loot, feel free to spend it all on your character.

Intelligence is only good for disbelieving illusions, Wisdom counts for resistance to charm spells and priestly spell bonuses, and Charisma is there to determine reaction adjustments and the number of henchmen. Your character is just as smart and charismatic as you are and thinks just like you do.

PCs, as pure engines of hack, don't need backstory or character development. Just be yourself. "Role playing" is a real misnomer; these are supposed to be combat strategy games.

Rules lawyer at every opportunity. It doesn't matter what the GM says, Appendix III.A.17 CLEARLY states that your paladin was right to slaughter those innocent villagers in cold blood. See, these righteous warriors of goodness are allowed, nay, required to put a godly smack on anyone who poses a challenge to the faith, and that one fellow in the striped suit with bells on was mocking you, so you had every right to seal the doors to the town hall and burn it down.

It doesn't matter how well you did if another player did better than you. Always be on the lookout for an opportunity to screw your fellow PCs over for your own benefit.

Your character knows everything you do and will act accordingly. This is especially handy if you've read the GM material as suggested above.

Addendum: Be a munchkin. You can make up for your genital deficiencies by creating the biggest baddest meanest ass-kickingest character possible. Give him a cool name like "The Amazing Bone-Crunching Engine of Death, Destruction, and Despair" to top it all off.

Always remember the most important point, which I'll restate: Winning is Everything!

Advice for both players and GMs

The Game and Real Life are inseparable! You are your character! Hold grudges over things that happen in the game world, and your group will go down the tubes faster than... something going down a tube very fast indeed.

Another addendum (thanks ^Davion^): Players: Try to beat the GM. Do your best to outwit him and foil his plans at every turn, as he is your worst opponent. GM: The players are A: stupid and B: the Enemy. Do everything you can to take the bastards down. Everybody in the group should hate everybody else.

Thanks to the nodeshell creator for the necessary inspiration. All the advice above comes from years of personal experience as both a player and GM. I have witnessed everything on this list at one point or another and done a fair bit of it myself.

Eh... this is rather D&D-centric, but that's what I know best.