In a recent article on how to avoid off-days during workouts in the Wall Street Journal, track and field coach Peter Thompson is quoted on the importance of recovery time. Specifically he states "you should train to recover, not recover to train."
Along with nutrition, rest is as important to your workouts as anything you do in the gym. Those of us that are passionate about exercise sometimes run the risk of over-training. When we impede our bodies natural ability to recover, more exercise gets us less results.
This is why its important to always plan a rest day after a few days of pushing your body, and also to make sure you get enough sleep. If you live an active lifestyle, odds are you need more sleep than the average person to heal your body.
You should also not be hesitant to take longer breaks from training, whether its because you are going on vacation or you just want to take a week and reflect on your approach to exercise and fitness. If you workout regularly over an extended period of time you won't lose much by taking a week off, and you might be surprised to find that some part of your body that had plateaued finally breaks out after a brief break.