First of all, the "how" of values affirmation; here's a great exercise to help you affirm your highest values: http://www.affirmvalues.com. And the "why"? As those of you who have attended any of my programs know, I stress the importance of discovering and affirming your values both as an integral part of harnessing your will to make changes in your life and of reducing stress.
For more about harnessing your will, see the first principle here [pdf].
For more about the role of values affirmation in stress reduction, read "Affirmation of Personal Values Buffers Neuroendocrine and Psychological Stress Responses" (Psychological Science) [pdf]. Abstract:
Stress is implicated in the development and progression of a broad array of mental and physical health disorders. Theory and research on the self suggest that self-affirming activities may buffer these adverse effects. This study experimentally investigated whether affirmations of personal values attenuate physiological and psychological stress responses. Eighty-five participants completed either a value-affirmation task or a control task prior to participating in a laboratory stress challenge. Participants who affirmed their values had significantly lower cortisol responses to stress, compared with control participants. Dispositional self-resources (e.g., trait selfesteem and optimism) moderated the relation between value affirmation and psychological stress responses, such that participants who had high self-resources and had affirmed personal values reported the least stress. These findings suggest that reflecting on personal values can keep neuroendocrine and psychological responses to stress at low levels. Implications for research on the self, stress processes, health, and interventions are discussed.
Or take a look at my post Your values: One way to lessen the stress of conflict (Brains on Purpose™).
Have you ascertained and affirmed your values lately?