Attitude innoculation (aka innoculation theory) is a technique used to strengthen someone's will against persuasive messages by initially exposing an individual to small or weak arguments that are against their stance or position. The individual, exposed to a small and practiced amount of the persuasive argument, will develop counter-arguments and a defense that can be used when actually exposed to the persuasion. Developed by William McGuire, attitude innoculation is based on the concept that individuals initially exposed to weaker arguments will build up a resistance to these persuasive messages. It is called innoculation theory because it is based on the concept of vaccination and innoculation - exposure to a small amount of the disease (or message) will allow you to build up an immunity and defense against the strong version later.
An example could be seen with the prevention of children using drugs. Using roleplaying with children about being asked to do drugs allows them to develop counter-arguments that can be used in real life if they are ever offered drugs.