A counter intuitive tip when it comes to fitness is to not overdo it. Perhaps you don't have the same problem as most people and you are super motivated and you go to the gym nearly every day. This is great, except sometimes you may push hard, end up injured, and not getting in any sort of fitness related activity.
If you are a runner, log the mileage on your running shoes. Shoes get worn out and when they do they can cause pain and injuries if they are still used. A good rule is to replace your running shoes every five hundred miles, even if they feel like they still have some miles in them.
Make sure you get plenty of sleep. Sleeping is essential for all life. While you sleep, your body undergoes repairs that it could not normally do while you are awake. Your heart rate is also lowered, and you are in your most relaxed state. This is important when working out.