I think of exercise and fitness as a way of life. I encourage my clients to think of it as any other scheduled aspect of their lives--skipping it is not an option, and therefore it becomes a habit. That said, listening to one's body is the most important: push when you feel strong, ease up when you feel fatigued. Having a balanced workout regimen of cardio, strength, core work, and other forms such as yoga or pilates (combined with a healthy diet!) will provide one with the best results, whether the goal is weight loss, strength gains, or life-long health and fitness.