For evil campaigns, I always find it useful to have good aligned PC's hire the evil PC's to fight off evil. Such as having the evil pc's in jail, the only way they can gain freedom is if they perform a task for the ruling council of the city or such. They can be ordered to track down an evil being even more evil that themselves. Or to explore a uncharted region of land and spy on the inhabitants. If the PC's gain freedom but refuse to carry out their orders, you can have the authorities after them, or bounty-hunter NPC's track them down everywhere they go. Another cruel element would be for the GM to have a wizard put a quest or geas spell on each PC.