One technique often referred to is the "foot in the door", after the infamous travelling salesman approach of getting inside the target 's house, because the poor saps who let you in (to demonstrate your vacuum cleaner or whatever) will do anything to get you out - including making a purchase.
In a famous social psychology experiment that illustrates foot in the door, psychologists go into a neighbourhood and ask residents if they would allow a poster in support of a local cancer charity to be hung in the front living room window. The poster is so large it would block out a lot of natural light. Not surprisingly, only a few residents say yes.
Next the psychologists go to a similar neighbourhood and ask the residents to wear a little badge in support of a local cancer charity. This is a small task and therefore a small "yes" for most people, so they agree to do it readily.
A few weeks later, the psychologists return to this neighbourhood. They now ask for that big poster to be hung in the window. This time many more people agree.
Theories about why this happens are many and varied. One idea is that once you start saying yes, it is much harder to say no. Another is that having accepted the badge, it does not seem to make sense to reject the poster. People like to be consistent, and agreeing to wear the badge means you complied with the principle of supporting the charity, so why now reject the poster?
This is an important psychological principle: apparent consistency seems to matter a lot to most people.
Another interesting idea is that now you have said yes to the badge, your identity is tied up with the cause. To reject the large poster is now in conflict with this newly established identity.
By starting with one small yes or more and building to a bigger yes, we are using powerful psychology, which makes it much more likely we will secure the big yes in the end.
An almost opposite psychological principle appears to be in play when experimenters go into the high street to ask passers-by to donate a pint of blood. Few people volunteer.
Next they start by asking people to commit to giving blood several times a year. Needless to say, very few indeed sign up. So the experimenter says: "OK, we understand this is a big thing, but as the blood transfusion van is just round the corner, why not just donate one pint today and we 'll forget the other rather demanding scheme?" Now many more people end up giving a pint.
Why is this? One theory is that it appears that the transfusion people have conceded the first request was too much and so are now asking for much less. There is a natural human tendency to reciprocate - once one person has made a concession in a negotiation, it seems only natural to reciprocate with a concession yourself: agreeing to giving a small amount.