Bussing is the act of distancing yourself from your scum partners, usually by helping to lynch them (and hopefully playing a major role in convincing others that they are worthy to be lynched). The term "bussing" comes from the phrase "to throw under the bus", which means sacrificing them to gain credibility. The 'bussee' is the player being thrown under the bus, and the 'busser' is the one doing it. The effectiveness of this technique varies, mostly due to the fact that it has been around practically since the beginning of the game of Mafia. The main reason it is successful (when done correctly) is because there is absolutely no way to know positively whether bussing has occurred, or whether the suspicion on the busser's part was legitimate, and coming from a Pro-town viewpoint.
Why bussing works
Bussing works because (if done correctly) it distances the players doing it to each other sufficiently that nobody (or at least, nobody but the smartest players) will ever suspect that they are both scum in the same faction. The reasoning applies that if the bussee is actually lynched; the busser is considered to be obviously pro-town. Because, of course, why would a Mafia member try to get his own scum partner lynched? And thus, WIFOM ensues.
How to spot bussing
Suspicion commonly arises when someone is suddenly lynched for what seem like minor or spurious reasons, and yet flips scum, because it may be indicative that one of the major proponents of their lynch is actually one of their scum partners.
If a scum player is about to be lynched (or feels like it might be a possibility that they will be lynched), another tactic is to bus their partner on their way down, trying to get them lynched as well. Often, this includes quite a bit of bussing on the part of both parties, but when the original scum player is lynched, his bussee is often (ill-advisedly) considered to be a guaranteed townie. The argument being: why else would the now-dead Mafioso have tried so hard to get them lynched? Again, this argument is WIFOM, and should be avoided at all costs. This tactic is similar to the double bus, except it is usually used in the context that one player is more likely to be town, and the other more likely to be scum.
When two (or more) players are the leading Bandwagons for the day, and they both happen to be scum of the same faction, they may (and often do) engage in double bussing. Both of the players begin to attack the other in any way possible, attempting to make their attacks appear to be an effort of self-preservation. This tactic is similar to regular bussing or reverse bussing, except that in this case, both parties are suspected to be scum, and neither has a significantly more credibility than the other. When done skillfully, double bussing is even known to cause the townies to dismiss suspicion of both of the bussing individuals, under the mistaken assumption that it is a town-on-town fight, and shouldn't be taken seriously.