I love playing antagonistic characters, from flat-out bad dudes to edgy 'dark knight' types within groups of shining heroes. I agree with a lot of what @Lisse24 said. I've usually recieved a lot of good words about how I play antagonists, so, I think I can provide some helpful guidelines.
On the player level:
*Clarify, check and disclaim - you need people to be comfortable around you and understand that you are playing a character. This means you may need to be more friendly OOCly than you normally are!
*Be willing to lose. If you're not doing this, then you have no business doing any sort of PvP conflict, even on a sandbox MU.
*Be willing to jump in to all sorts of ideas. Your antagonist needs to be able to do multiple things and you, the player, need to be able to quickly think on your feet to come up with reasons why they might be, one day, robbing a bank and, the next day, fighting for mutant rights and, the day after that, trying to blow up New York, for example.
*Your antagonist has a shelf-life and will need to be defeated at some point. An intense antagonist who does a strong 2-3 months of RP and goes out with a bang will be remembered more fondly than one who exists for years and does very little beyond seem to be a prick to people.
*What does your antagonist want? Are they willing to compromise? Every antagonist is just the hero of their own story.
*Dangle hooks in front of the more heroic types. Whether that's the secret of pushing through your anti-hero's edgy exterior, or the first seeds of defeating your evil villain, dangle those damn hooks with the fury of a thousand suns because a lot of good guy players are incredibly dull and need to be led like a horse to water. A lot of my antagonism hinges on a bit of clever, fun 'metaposing'. The Dark Lord laughs behind his helm, glowing scepter held aloft! "With this power, I will be invincible!" The power that radiates from his incredible power gem at the head of his scepter is incalculable, but, surely not invincible! If only he wasn't holding that scepter...
*Similarly, build plots around players. Reach out to players and find out what they want or might be looking for. Maybe there's someone who wants to have an NPC taken hostage, or someone who wants to get badly beaten down, or someone who wants to put their character through a test of their character, but you need to do the looking.
*This one is more for anti-hero edgy antagonist heroes, but: have an off-switch and an understanding that you can't be on all the time. A lot of the time, you need to shut your mouth unless your comments will heighten the drama or be particularly relevant. An anti-hero with a sarcastic biting wit can be a lot of fun, but it becomes less fun when they're always doing it, day in and day out, and it seems like it's a vehicle for the player to be a snarky asshole.
A lot of it is very similar to what @Ghost said, really.