Pull ups are very beneficial, but you should take into account the following safety reminders in order to avoid injury.

1. Do Not swing or bounce when using your pull up bar.
2. Do Not attempt to hang by your feet or legs from your pull up bar.
3. Do Not use a doorway pull up bar if you weigh over 300 lbs.  Try a pull up station instead.
4. Do Not use a doorway pull up bar if you are unsure about the stability of the doorway or door jamb.
5. Do Not allow children under 18 years of age to use a doorway pull up bar unless supervised by an adult.
6. Ensure that you test to make sure your pull up bar is completely secure in the doorway before using it.
7. Make sure you maintain a controlled motion when performing pull ups or chin ups.

Pull ups and chin ups are great at-home exercises, but you need to take these precautions to ensure that you avoid injury.  Now get back to your workout and add some reps!

In this 12th video of the series, fitness instructor Danielle demonstrates how to use your doorway pull up bar for sit ups. By placing your pull up bar at the bottom of the door frame, you can place your toes beneath the bar to ensure you are using proper form for your sit ups. The bar should not lift when doing sit ups. It it does, you are using your leg muscles too much for momentum, and not using your abs effectively.
Danielle explains proper form for sit ups, which is important for avoiding injury, and for maximizing the benefits of your sit ups. This same method can be employed for crunches as well. Ensure that your lower back and shoulder blades are making contact with the ground, and make sure you are using a fluid motion.
Watch the following video as Danielle demonstrates proper form for the most effective sit ups and crunches.

Fitness instructor Danielle demonstrates how to perform push ups using your doorway pull up bar.  It is important to have a variety of exercises in your workout plan to ensure that one muscle group does not get overworked, giving you an unbalanced appearance.  Danielle demonstrates and explains how narrow grip push ups and wide grip push ups will target different muscles giving you the balanced appearance you are after.

Watch the video below:

Hanging a P90X Pull Up Bar on your door frame is simple. Danielle demonstrates this in the second video of the instructional series. This pull up bar will fit up to a 32" door frame and trim is required. See the video below.