Normally, you're pretty upbeat. But once in a while, your mood hits the skids and you become tense, cranky or down in the dumps. What's going on? The truth is, moods hinge on a precarious interaction between your physiological state, your thoughts, environmental influences and life events, according to Robert E. Thayer, professor of psychology at California State University in Long Beach and author of The Origin of Everyday Moods.

To perk up your physiological state, be sure to get enough sleep, eat nutritious meals and exercise regularly. Then brighten your thoughts with these simple strategies to kick a bad mood to the curb: 1. Daydream.

Researchers at Stanford University found that reflecting on good memories when you're in a bad mood can activate different parts of the brain that can influence mood in a positive direction. 2. Get in control.

Organize your desk. Clean out your purse. Tackling a doable task will make you feel more in control, which can lift your spirits when a situation is dragging you down, says Thayer. 3. Chat up a stranger.

While waiting for your latte or standing in the checkout line at the grocery store, talk to someone near you. Acting extroverted -- being talkative, assertive and adventurous -- makes people feel happier, even if they're not naturally outgoing, according to researchers at Wake Forest University. 4. Give thanks.

Jot down a few things you really appreciate in a journal -- the dinner your spouse cooked last night or how your pooch always makes you smile -- and your mood will quickly take an upturn. "Gratitude has no negative side effects -- it feels good to experience it and express it," says Michael E. McCullough, professor of psychology at the University of Miami and author of Beyond Revenge: The Evolution of the Forgiveness Instinct. 5. Stand tall.

Operate on the act-as-if-you're-happy principle until the feeling comes naturally. Research from the University of Tampere in Finland found that when people adopted a genuine smile, they started experiencing feelings of pleasure, regardless of how they felt previously. So smile, stand tall and relax your body to reflect a sunnier, easygoing disposition, and you'll help yourself turn a bad mood into a good one.

Stacey Colino has written for The Washington Post's health section and many national magazines, including Newsweek, Real Simple, Woman's Day, SELF, Marie Claire, Cosmopolitan, Glamour, Parenting, Sports Illustrated and Ladies' Home Journal

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