The main thing cults do is take a common belief and spin it. Look at the "God hates Fags" website, for example, that is a cult.

In essence, this is how a cult operates.

They first try to get you in by giving you a free trial sort of thing, an offer that doesn't seem so much and isn't that much of an inconvenience to you.

You start off small at this point, and most of the things you hear aren't too radical, thus making you more inclined to join.

If you join, you often go through an initiation period which they use to indoctrinate you into the cult. Usually it involves a great deal of work or ceremony, including but not limited to changing one's dress and diet. This further enforces the "you were once one of them, now you are one of us" idea. One of the reasons fraternities have such rabid loyalty is because they operate along the same basic prinicples. The terrible hazing that a person undergoes solidifies their belief that such a thing is good. As we are horribly self-biased (we tend to see ourselves as above-average intellect, for example) we believe that any group we join MUST be good because we belong to it. Also aiding this is the fact that we think that if we've gone through so much hardship to join the group, then it MUST be good. These two principles combine to make a rather powerful magnetic affect, in essence keeping you in.

Once you're in, you will often be put to work to be distracted from "unthinkable" thoughts, i.e. "man, this is stupid". This, coupled with other things such as chanting and song, meditation, and constant group interaction keep you from thinking that it isn't a good idea, and once you do get around to thinking about it you tend to think it is good because you wouldn't have stayed so long otherwise.

As your faith becomes stronger, they can up the dosage and introduce you to more parts of it, often with harder tasks involved. The more you are willing to accept, the further in you are drawn.