Keeping fit

It's a losing battle, it seems. Keeping fit, staying in shape, fighting the "flab" (= staying slim) – we know we're supposed to do all this, but what actually works? We're often told that crash diets (= very severe, short-term diets) don't work, and that regular exercise plus healthy eating habits are the only way to maintain ideal body weight. What do you do to stay fit and healthy? I'd love to hear your secrets!