Most important: Let the players know that it is an amoral campaign ahead of time.
Our DM started a D20M campaign but didn't have much of a story thought out ahead of time. As a result he told us to create our PCs, but wouldn't tell us what kind of campaign we were playing, other than that there would be no magic.
Anyway, I created my PC as an "Average Joe". He customizes cars for a living, basically he's a mechanic (Fast/Smart, Blue Collar). He also happens to be a normal, good, law-abiding citizen. Then I find out that our campaigns will consist of acting as hitmen, being hired to kill innocent people, that sort of thing. His campaign is so linear that it doesn't really give us the choice of doing any missions but the ones he has sent us on.
The other PCs include an actor in martial arts movies (Fast, celebrity), a guy who was kicked out of the military after training for some reason (Strong, military), a college professor/lecher (Smart, academic) and a hitman (Smart/Fast, criminal). With the exception of the hitman none of the PCs are "evil", or even criminal.
The celebrity and the military guy are played by younger players who have a harder time (or they aren't trying) roleplaying their PCs. This means that they tend to shuffle their feet and twiddle their thumbs until the DM forces them to do something, or one of the older players' PCs gives them a job to do. On the few occassions they think up their own plans... Well the results are frightening to all but the most heavily-medicated catatonics. These two are equally at home being evil or good, because they don't really get into their characters, nor do they particularly care to. Their PCs have no personalities of their own.
Since my PC is essentially good, I often try to worm my way out of our missions, the hitman tries to complete them as told to, and a battle of wills ensues. Whoever seems more confident in his plan essentially captures the "votes" of the younger players and "wins". Since the Hitman's player is neither confident, nor as familiar with the rules as I am, I can usually sway things my way. As a result we have failed to complete any of the jobs we were hired to do except one, because it involved being hired by some Yakuza to kill a rival gang leader. My PC had no qualms about offing a murderer and so we had no problems. My PC was nearly killed though and when we left off there was a sword cane stuck in him, so who knows what will happen.
Our first job was to rob a member of the russian mafia for a member of another crime family. We failed that one because the DM had planned for us to fail, and then railroaded us to that end.
We fled the state and got another job in California. A man, who we later discovered was selling drugs, had hired a man and woman to *ahem* perform for his viewing pleasure and then he killed the man. When we arrived at his house we discovered that the job was to dispose of the body and kill the woman, who was still alive. My PC was horrified by this, and despite almost being thwarted by some Police that inexplicably dropped by, and a group of heavily armed SWAT team look-alikes that stopped by for drugs and almost killed everyone, managed to form a plan to kill the guy that hired us and free the woman without her turning us in. It worked and we failed job number two.
Anyway, I've rambled too much already.
Just make sure the players know what the campaign is going to be like before they make their PCs. Having at least a general idea about what the campaigns "story" will be is also helpful. Nothing makes a game go flat quicker than a DM that has no clue what the story is.