Self-Affirmation Technique in a Nutshell
Self-affirmation technique is invaluable in public influence, social advocacy, political persuasion, or whenever you have to deal with issues that provoke people’s ego-defensiveness. As a rule, it is best not to try changing people’s minds about issues threatening their ego, especially their deep-rooted, strong political or moral attitudes and beliefs. However, if you will try changing opinions about ego-involving issues, self-affirmation technique should be on the top of your list. The basic idea here is to strengthen people’s self-concept (popularly known as ego), for example by asking them to reflect on their important values, before you challenge any of their attitudes that might provoke ego-defensiveness.
How Self-Affirmation Works
This technique is based on the understanding that people derive a sense of control and predictability from a stable self-concept. It is difficult to change people’s opinion on specific issues because by attacking a particular attitude or value, you also threaten their overall self-concept, so people naturally become defensive. Self-affirmation exploits this underlying dynamic: before challenging a particular attitude, belief, or value, you strengthen people’s overall self-concept. You can strengthen self-concept, for example, by providing them with a positive feedback or by asking them to reflect on positive values of their self-concept. Now, with the stronger self-concept, a person is more willing to give up a particular attitude or belief. (As you can see, self-affirmation technique here has little to do with self-affirmations of the self-help industry.)
Self-affirmation and Threatening Messages
Studies show that, among other things, self-affirmed people become more open to threatening health messages. For instance, women who drink plenty of coffee are usually resistant to health messages linking caffeine to breast cancer; however, self-affirmed women are more likely to accept such information and change their behavior.  Likewise, health messages about the risks of contracting HIV normally threaten the self of sexually active people, but self-affirmed individuals become more open to such messages. 
Affirmations Unrelated and Compatible with the Message
Self-affirmation tactic works only when the affirmation is based on values that are either unrelated to your message or related but compatible with it.
A 2004 study by Julia Zuwerink Jacks and Maureen O’Brien tested this tactic with a message arguing that gays should not be allowed to serve in the military. The subjects, however, were all in favor of gays in the military.  Before the subjects read the message, the experimenters asked them affirm their self-concept. Some subjects used self-affirmations unrelated to the message: they had to describe three examples when they succeeded at being honest. As the researchers predicted, thinking about honesty strengthened their self-concept and they became more open to the message which contradicted their favorable attitude towards gays in the military. The other subjects, however, used a self-affirmation related to the message but incompatible with it: they had to describe three examples when they succeeded at being non-prejudiced. Of course, being non-prejudiced and opposing gays in the military are incompatible. So predictably these subjects did not become more open—in fact, they became even more resistant than they were at first.
So for self-affirmation to be effective, you can use either affirmations unrelated to your message or affirmations related but compatible with the message.
You can use one of the two methods to affirm people’s self-concept: you can ask them to generate various reasons that would strengthen their self or you can provide them with a positive feedback yourself.
Affirmation through Self-Persuasion
Self-generated reasons are usually more effective than any reasons offered by other people, and self-induced affirmation is no exception. So whenever you can, it is best to use this method: ask them to reflect on the positive aspects of their self. In particular, you can ask them to recall instances when they upheld a value or a principle that they care about deeply.
Affirmation through Positive Feedback
When you can’t use self-persuasion approach, you can provide them with your positive feedback and so strengthen their self-concept. This method is similar to basic flattery, but you should find something they truly value, something that is an important part of their self-concept. Some people may care more about such values as honesty, dependability, sincerity, and fairness. Others might value more benevolence, caring, kindness, helpfulness, empathy, and humility. Others yet might care more about spirituality, religiousness, traditionalism. So if the most important part of their self-concept is religiousness and traditionalism, it might be ineffective to affirm their fashion sensitivity or rationality or critical thinking.
- Mark B. Reed and Lisa G. Aspinwall, Self-Affirmation Reduces Biased Processing of Health-Risk Information, Motivation and Emotion, 22, 99-132 (1998).
- David A. K. Sherman, Leif D. Nelson, and Claude M. Steele, Do Messages about Health Risks Threaten the Self? Increasing the Acceptance of Threatening Health Messages Via Self-Affirmation, Personality and Social Psychology Bulletin, 26, 1046–1058 (2000).
- Julia Zuwerink Jacks and Maureen E. O’Brien, Decreasing Resistance by Affirming the Self, in Resistance and Persuasion, Eric S. Knowles and Jay A. Linn (eds.), (New Jersey: Erlbaum, 2004) pp. 235-257.