[Oskatat]

For an evil campain:
Let the players know, thoroughly, that there is evil and evil:
Evil for the sake of evil (psychopathic mass murderer)
Evil to accomplish a (evil) goal (ruthless murder of a competitor)

If an evil party comes together, make sure they have something in common, like a common goal, to keep them cooperating, plus someone who would toast/fry/squash/break/eat them if they fail or turn against each other. They are evil, so friendship might not work (loyalty, friendship and love are still possible!!! not all evil characters are demon/devil like), but fear should do the trick just fine. And in the end they can go kill this master who's been terrorizing them together (unless its a god).

*****
I had fun with a drow "lord" (lady in fact) who charged the party to gather more 'power' for her, like an army or strong adventurers. She charged them not to squable and had an army of drow, drow assasins and other connections to find traitors of her cause, and killing (one of) the others was betrayal. It happened once, but the players learned from his fate (graphic description of his fate only available for players and DMs above 18).
*****

One other tip, dont get upset if players correct you. You are human, you make mistakes, you learn from mistakes. How would you know you made a mistake if one of the players wasnt kind enough to point it out? This excludes smarta$$ing