Activism begins with a commitment to speak out on one or more issues consistently. To maximize our effectiveness, psychology and identity must be taken into account.
Psychology
Studies have demonstrated that repetition creates the impression that an idea is widespread (thereby sparking social conformity and contagion) even if the ideas in question originate from a single source.
In any given population you will find very few people that truly think for themselves. Most are carried along by social conformity or follow the lead of a supposed authority. The Asch conformity experiments, and the Milgram authority experiments demonstrate this clearly.
If the forces of conformity and authority are currently holding humanity on a destructive course, it is up to us to turn the tide. The Asch experiments demonstrated that even one voice of dissent is enough to break the power of misguided conformity (the emperor wears no clothes).
Ideas are more likely to take root when they are voiced authoritatively, and repetitively. Multiple voices working in coordination can amplify this effect, and create waves that take on a life of their own.
Identity
Most large cultures comprised of multiple subcultures. Within any given western country you will find a myriad of religious, political, and ideological identities (as well as countless identities formed over trivia such as musical tastes, or sports teams). These identities heavily influence the way ideas are perceived and integrated.
Effective activists take the various group identities of the target audience into account at all times and tailor their message to avoid triggering cognitive dissonance. This often implies using old words for new ideas, or new words for old ideas.
Example: the term “sustainability” has a negative connotation among conservatives in the United States, while “local resilience” does not.