The most neglected part of many fitness programs is rest. Progress from training occurs during the recovery, not from the training itself. Your body repairs and strengthens during rest; continuous training can actually make you weaker. If you workout every day you need to create a routine that allows your muscle groups and energy systems to recover.
This does not mean you need to rest every other day, but you should cycle your workout to ensure this repair process. This is why many lift weights every other day, or split their routine to work different muscle groups on different days. Cross training is another great way to aid your body. Running and swimming are great aerobic complements, as they each stress different muscle groups. If you do high intensity workouts then a rest day is even more essential, some may need more than one day to recover. For those who feel too guilty to take a day off, aim for active recovery after a high intensity day. This means an activity that is easier for you, for example if you ran some serious hills, take an easy jog on the flats.
For athletes and those into serious training, periodization is highly recommended. This is a structured program that can be up to a year long. It breaks training into various cycles that can include preparation, competition and transition. This allows for maximum preparation and rest to avoid injury and overtraining. You can do this with any activity. One example is to lift heavy weights for strength one cycle, and for endurance (lighter weights with more reps) on another cycle. However you choose, just make sure to give your body adequate rest (including sleep), and it will thank you by showing you better results.