Let's try this again. My first reply disappeared on me....
Anyways I am currently DMing an evil campaign and we are about to hit level 4. I have lots of advice for you.
Have the players sit down and discuss character concepts. Discuss what kind of evil campaign it will be...is it random acts of evil, is there a plan or a goal and maybe even more importantly how will they go about acheiving it. Also discuss what type of evil is allowed. Should party backstabbing be acceptable? Will they abandon the party to save their own skin? Will they kill the high priest if they can get away with it?
The current campaign I run is using Waterdeep (and it will mostly be a city campaign from start to finish). Most of the players are followers of Cyric. We even have one player who has chosen to play a Paladin of Torm and the first 5 levels are about the party causing his downfall to a Blackguard. Every one talked about this and agreed this would be pretty cool to kick of an evil campaign. I have given the Cyricists a magic item that foils the Paladin's detect evil ability and they have all taken skills in disguise, bluff and that sort of thing to masquerade as a follower of a different diety for now. The crux of the paladin falling is that he is a lower born noble of his family and the family is known for making superior weapons. Recently, a rival family have suddenly been producing superior weapons at a fraction of the cost. The head of his family has asked him to look into the situation. He actually sought out mercenaries (the rest of the party) to help in uncovering what is happening.
It turns out the rival family has been purchasing mithral laced ore from an unkown source at a deep discount. AFter finding the source, they discovered it was Duerger dwarf who is trying to dump ore on the market to help finance a war with Menzoberranzan. This is where things get tricky for the Paladin. If he does nothing, his family suffers. He could stop the duerger but another could just as easily come along so that doesn't help either. Instead, he brokers a better deal with the duerger to supply ore to his family. Sure, the duerger race is evil but as far as the paladin knows this particular one has not caused any harm or broken any law. Better to have him in his pocket where he can be watched and dealt with if need be. Besides who's to argue if two evil races want war with one another.
Unfortunatly, for the Paladin, I have spoken to all the players outside the game to help reinforce this point of view and so he went along with this.
The next step puts the Paladin in some serious trouble. The law (aka town guard) discover this deal between this noble family and the duergers. If this knowledge is exposed to the Lords of the town then not only would there be an investigation but the family may lose their nobility and at the very least suffer a massive penatly to their reputation. This is again where the rest of the players come in, they encourage the Paladin to stop this information in coming to light. Likely it means killing a few law enforcement officials to cover up the secret. And, voila, the paladin falls....peronal downfall for the good of the family.
All this was accomplished because we sat down and discussed the campaign. The paladin player knew his character was going to fall, but now how, and that was the goal of the rest of the party.
As DM I have also stressed that there will be consequences for their actions. They could commit any evil the wanted but the more public it became the more they would find difficult foes looking for them.
In addition to this, I have statted out 1 evil party and 1 good party that they may encounter throughout the campaign. Early on they meet some of these guys individually and then later on as an NPC group. I will also change the dynamics so that sometimes they are allies (due to circumstances) and sometimes they are enemies.
Oh, one last thing. I have also stressed that not everyone is out to kill them and thus they should act in the same manner. This helps make reoccuring NPC's and if they don't heed this then these are the reasons powerful NPC's may begin looking for them.
So far so good.
Hope this helps and hope it inspires your campiagn.