A lot of Policy is driven by a thorough understanding of this principle. And I mean, thorough. Unfortunately, People Will Do the Easy Thing is discouraging in that it does not lead to the desired outcome. Here are some examples:
(1) Fitting a diaphragm requires skill for it to be effective. Using a diaphragm effectively requires effort and skill. People will do the easy thing. They won't use a diaphragm. Try something else. Condoms require less skill, but some effort, and there is a cost (time/resources to have them available, some cost in sensation, some transaction cost in getting partner to use them). People will do the easy thing, etc. Down this path lie IUDs. Unfortunately, turning the IUD into something that requires little skill increases the hazard. And it turns out they just aren't that effective over populations.
One of the problems with going the Easy route is that you might choose something dangerous (and blind yourself to the risks) or that doesn't get you what you want, because it is easy. Bureaucrats are people. They choose the Easy thing, too.
(2) It's hard to convince people who do not buy or sell sex and who do not inject drugs that it is worth spending money on people who do buy or sell sex and who do inject drugs. If they are in jail, it's even harder. It's a lot easier to convince people to spend money on "innocent"/"good" people. This is why you wind up down the spend-money-on-development path and/or focusing AIDS awareness on the not-sleeping-around spouses of the Wicked Injector/John. Unfortunately, while it is very easy for a scientist to make the argument that this is a stupid waste of money, the scientist, after a decade, will learn that people are better able to memorize the argument for/against focusing on the innocent *rather than actually doing the Right Thing*. Scientists are people. They do the easy thing, too, which is keep saying, hey, this works, that doesn't. Knock off what you are doing and do it this way instead.
One of the waria Pisani talks to says Men are Lazy. They won't go buy a condom and bring it with them to the waria when they are going to buy sex, because they are too Lazy. But if the waria is saying put this on after the man is all hot and bothered and ready to play, the man is too Lazy to argue. He'll wear it.
What Policy needs to do is identify the people who are motivated enough to _not_ be Lazy, who are willing to do something _other_than the Easy thing and/or whose calculation of what is Easy turns out to also be the right intervention. And then figure out a way to replicate that. That, however, is Not Easy. Pisani has brilliant examples of this, for example, making it the brothel-owners responsibility for the people in the brothel to use protection, and create a reasonable enforcement mechanism (e.g. clinics that treat STDs find out where the johns have been buying sex; test sex workers, etc. -- shut down brothels that are transmitting disease). Unfortunately, I don't think you can "cookbook" this kind of Policy -- it is all very situational.