Gotta say, running an "evil" game, and having done so once in the past, Lapak's idea illustrates an important thing to remember when running this kind of game. It's the same as any other type of game. Find out what motivates the players and give them something to do that is related to that motivation.

Alternatively, you can do what I've done with my current "evil" group. Make them part of a larger organization where they have independence, but still have "bosses" that must be answered to. Occasionally, they get sent on a mission of some sort. Eventually they figure out what they want to do and begin pursuing that goal. Then you just figure out a way to use that goal to motivate them into a plot-line or quest.

I tend to stay as far away from railroading as I can and give the players as many options as possible. This gives them the false sense of having a choice.