"You can't con a good man" is what some con artists would say. While that's partly true, the more subtle and insidious acts involves good people.
Take for example loaning money. Who in our entire lives had no need to borrow money from someone else? The unscrupulous person would scam his way out of paying his debt, but the honest person would strive to pay in full the amount he owes the other person.
Long-term debt is an insidious way of taking advantage of a person. If I (pretending to be an evil megalomaniac) were to loan out a large sum of money over the long term, there are two methods to "earn" from this. One is to charge interest. The other is to gain leverage, but this kind of leverage is only possible through honest people. I mean selfish people would probably mutter "he's stupid since he's not charging interest that I'm willing to pay". The honest person, on the other hand, think that I'm such a good friend, and that he'll owe me a favor or two as a means of repaying the lack of interest (without mentioning it to me, of course). The other person might even be mad at me in the future or wants to pass on a profitable proposition to someone else, but since he owes me money (or owed me money at a previous time), he obviously can't take full advantage of that situation. He might behave himself when in front of me even when he's mad, or a business venture was presented to me instead of his other friends.
Money is concrete. When you owe someone something abstract (such as good will), it's easier to take advantage since there's nothing definable, and you're left with your conscience to put a price on the abstract matter. And whether it's humility, wishful thinking, or a respect for the social norms, good people usually overprice this intangible debt.
In this example, let us pretend I am really an evil person. Because the actions I am about to tell are things I really do, although the motivation is not (at least not too much, hehehe).
Example #1 involves treating people out. I can treat them to lunch, dinner, or merely buy them a gift. To supplement this example, the said person in question is someone I barely know. A selfish person would merely smile at my actions and would probably only associate with me in the future with the hopes of getting more freebies. However, a scrupulous person would think that I'm a nice guy, and would behave kindly towards me in the future. Some might even think that I must be repaid for my efforts, and this can take the form of free lunches in the future (at the cost of one free meal, I get several free meals in the future), or simply favors (such as introducing me to a friend of theirs that's my crush) that they wouldn't perform for people on an ordinary occassion.
Example #2 involves loaning something to someone you want to see, such as your crush for example. I could loan the person a book, for example. I immediately get two rewards for this. One is the sense of debt the other person feels, and while it isn't as much compared to giving a gift or treating them out, it's still there nonetheless. The other reward is the fact that I will have to meet the other person again, since they have a possession of mine with them. The latter, in fact, can take the form of a demand. I mean I can easily make an excuse of wanting my book back to force a dinner date with the other person ("you can return the book to me at tomorrow's party") for example. And unlike the free gift in example #1, I can choose and demand the reward I'm going to get (i.e. "Are you done with my book yet? Don't worry, you don't have to return it yet. Although it would be greatly appreciated if we could meet tomorrow for lunch perhaps. Or maybe you could loan me that comic of yours."). It gets even better if the said person is unable to return the object that you loaned, be it because it got destroyed while in the other person's possession, or it simply cannot be found (as is the case with many people who have messy rooms). The said person can either stall (in the hopes of the object popping up sometime in the future) and get stuck doing perpetual favors until the said item is recovered, or admit the loss, and make various promises and reparations (which translates to more good will and a sense of debt, especially considering the object wasn't given freely, and the loss of it wasn't planned [which is the case with gifts given freely]).
Example #3 involves making a good first impression. If I'm kind and generous the first time you meet me (and a few times after that), the other person will think that I am a good person at heart, even if I do selfish acts in the future. The person would always be reminded of the way I acted at the start, and will most likely think that I'm still that person, even though I've clearly changed. At the very least, I get the benefit of the doubt, and I can reap the rewards in case the said person rises to a position of power (such as a poor person suddenly becoming wealthy, and perpetually treating me out to lunch because I treated him out on our first meeting).