First off, we're going to need some definitions. Evil is not rampant destruction for the sake of destruction. That's just retarded(even ragnarok has a purpose). Way too much wasted effort. EVIL IS ACTIVELY CHOOSING SELF SERVICE WITH THE COST PAID BY SOMEONE ELSE.
In the interest of organization I've laid it out in the format of "The 7 fun things" er, "7 deadly sins".
- Lust: We all want to be surrounded by pretty things. If you're evil though, look at your character sheet. Is your intelligence greater than 7? If yes, then you're smart enough to realize that there's a cost to everything you do. If you want women that badly, anything and anyone can be bought. The trick is figuring out the price, and getting someone else to pay it. Being evil isn't a license to go a-raping or go on a murder spree. Oh sure, you CAN go do stuff like that, but there's a better(and often easier) way of doing things. And the easy way is what it's all about. Always remember that people are people. No matter how uptight, strong, and self-righteous that paladin chick is, if she's one of your companions, she's eventually going to be at your mercy. If you have no more use for her, that's the time to bargain. Remember to start small. Don't be an idiot.
- Gluttony: This is one of those things that I think most DMs have almost no problem letting an evil PC shine. I don't know of very many DMs who try to dissuade players from spending money on more than just the bare necessities. Heck, this is one of those sins that pretty well defines most of the people I've ever met. This isn't about overeating, it's about lack of moderation. It really shouldn't be called gluttony. It should be called over-indulgence. Whatever your poison is, crank the dial to 11. Be it food, women, power, even violence. This is your characters element, excel at it. Live, eat, and breath your obsession. In an evil character's mind it's passion, not obsession, and it's taken to the max. That it hurts the innocent . . . well, you can't make an omelette without breaking a few eggs, and nobody is an innocent.
- Avarice/Greed: (sigh) Ok, this is one that gives your DM and parties trouble with evil characters. It's also where a lot of people screw up in playing convincing evil characters. If your character has an intelligence greater than 7, you're smart enough to realize that there's significant risk when you steal things. If you're stealing from your party while they're still alive, you're asking for trouble. This isn't a motivation for little stuff. Tiny gains are rarely net gains. What's a 500 gp ruby in the grand scheme of your multi-billion gp career? Greed is your motivation for the big stuff! Conquering kingdoms, bringing about the end of the world, blackmailing the captain of the guard with the knowledge that you know where his bastards are. If anything else, this is the one your DMs should love you for. Just make sure you have an active dialogue with your DM and the other players as to what you would LIKE to do. Be prepared for a bit a of backlash from time to time. Just because you're greedy and a two bit crook doesn't mean there's no room to negotiate.
- Sloth: Almost every openly evil individual is a lazy procrastinator. Why choose to put off enjoyment when you can have it now? After all, given the average lifespan of evil in most adventure games, you're not likely to be around later with all those blasted paladins running about like tin-plated headless chickens using their goody-two-shoes stink-eye on everyone they meet. This is where you have to decide if you're going to play this character or not for the long haul. It's easy to be a useless jerk in the short term, but here's the thing that every successful evil person and dragon and fiend has ever known: useless jerks don't live long. Sloth is for when you're powerful. Sit back and watch the slave girls dance while feasting on mutton all you want, but when there's knife work that needs doing, get to it! This isn't to say that you can't be slothful during your adventuring career. Only, don't call it that. Instead, be efficient. If evil people worship anything besides themselves it's efficiency. If you must do something, do it in the manner that deals with it once and for all with the least overall effort. If that means you have to deal with those smelly paladins, then by all means do so(though, it might not hurt to see if you can't get rid of paladins AND the problem in the same stroke).
- Wrath: My very favorite on the list. Where the other 7 deadlies are all the will, wrath is the sword that carries out your will. Being evil isn't about being a jerk. It's a gamble that your will is stronger than that of everyone elses. It's a contest. And in this game, evil plays to win. Which means using tools that are considered to be too dangerous by others. Make no mistake, wrath has a cost, but to the evil character, it's a price that is worth paying. For those who have perhaps partaken once to often of this means of attaining ones goals, it may well be all they have left. Wrath can be exceedingly hard to RP effectively without looking like just another muscle-headed goon in most adventure RPGs. This is partially because wrath is a luxury. It's easy to be wrathful when you have power. When you're not powerful however, it just tends to make you look stupid. Always keep that in mind.
- Envy: This one is sorta like greed. Whereas greed is a pursuit of wealth, status, power, etc. Envy is the desire to take the wealth, status, etc. from others. Fairly certain that describes the motivation behind every theft and the vast majority of wars in history. In game, the best way to use envy is to keep it directed outside the party. If you absolutely feel that it is imperative that your character envies someone in the party, AS PLAYERS work something out together. Maybe there's a gem or magic item that one of you managed to acquire. Perhaps have the two players decide to use the item as a contest, each one trying to one up the other as they continually steal it from one another, even an evil character generally wont cross the murder line over the item if he realizes the rest of the party will know it was him. If one side does eventually win(generally by orchestrating the death of the other), it's a contest both PLAYERS agreed to, and there's nothing stopping the DM from allowing the deceased to pull a trick out of a day time soap and returning to collect vengeance. If you're going to take a risk in a magical world, be aware that there may be magical repercussions.
- Pride: Ah, the sin that every blog writer can be accused of. The sin of every would be hero or villain. The belief that you, or anything about you, actually matters and that you are entitled to being remembered in some way. At least, that's how it is for normal people. Like many things, an evil character often takes this one a little far. A successful evil character will learn to keep it in his pants though. Just because you regard the rest of your party as walking talking armor and your future slaves does not mean you should say it. And you know that.