Are you anti-safety?
Signing the petition seemed pretty innocuous, and most people would not be too opposed to identifying as pro-seat belt and pro-safety. With this simple action, however, those who signed in their minds began to identify with this cause and began to see themselves as agents for it. Because of this decision and the personal identification which took place afterwards, the second request did not seem terribly ridiculous. When approached and asked if they would put this sign up in their front yard, those asked did the mental equation of "I'm in favor of safety" and because of it were much more likely to agree to the large sign.
How This Can be Applied
With the California prop voting coming up in November, there are several special interest groups that will be trying to garner popular support for their proposition going in to the voting process. For the sake of conversation, let us look at one of the the most polemic of these propositions: legalizing marijuana (Proposition 19).
Now how could Cialdini's identified phenomena be used to actually make the public more favorable to this proposition? One methodology would be to first approach potential voters in a canvassing effort and request that they sign a pro-free choice or pro-freedom petition. Reasons given for the request could be that the group is trying to help the people show the government that they feel that citizens should have the ability to make free choices without government interference.
Presumably, with this decision to sign the seemingly innocent "pro-freedom" petition, individuals approached would be linking themselves in their minds to being pro-free choice and as agents for this cause.
When approached, ideally the next weekend, by another canvassing group asking them to sign a petition saying that they feel people should have the right to choose whether to smoke marijuana on their own without government interference, the suggestion should be couched in terms of freedom and personal choice. The jump could potentially be too large for many to make, between "freedom" supporter and "freedom to smoke marijuana" supporter, but I would guess that one would receive much more positive responses to the second request after the initial one.
The Approach Can be Two-Sided
Continuing with the marijuana proposition, opponents to prop 19 could have a two-tiered canvassing approach of first approaching potential voters and asking them to sign an "anti-drugs" petition. This seems incredibly innocent and simple, and I would be surprised if there was too much resistance to it.
By signing the petition, individuals would be creating a link in their minds between themselves and the anti-drug cause. This could be exploited in a proceeding canvassing effort with the petition to be anti-legalized marijuana.
Why Do People Need to Sign Something?
One of the important items in Cialdini's book is the associative significance of signing your name, and even better, if your position is to be visible by other members of your community. While simple, the act of signing your name creates a very strong link in your mind, much stronger than a simple verbal 'yes' or 'no'.
While it involves more time and resources, the two-staged canvassing process can be used to create supporters where there were previously none.