If you feel like you've been overindulging on booze and sweets lately, or snacking mindlessly while staying up late, you're probably ready to slim down and tone up. Or perhaps you've been finding it difficult to stay motivated and active. These combinations often slow down your metabolism and make you feel sluggish and soft. What exactly is your metabolism? Simply put, metabolism refers to the chemical processes by which your cells produce the substances and energy needed to sustain life—and the higher it revs, the more energy (and thus fat and calories) your body burns. Naturally, it tends to slow as you age. You can also be passed down a less efficient metabolism from a parent. But you can actually trick your body into burning calories more efficiently. Here are a few sure-fire ways to not only fire up your metabolism, but keep in the red all day long so you can look and feel your best.