I personally don't believe in "back exercises", as I've noticed that people eventually injure themselves in time. When I did competitive level gymnastics college, & we used to do an exercise called a "log roll" which develops the body's muscles to work in unison for control, which is crucial for learning bascially all gymnastics movements. This static routine worked both the ventral & dorsal sides, & is perhaps one of the most fundamental conditioning methods; it was nothing more than as follows:
Note: This exercise is not for beginners or the faint of heart. If you aren't in shape & in particular if your midsection/abdominal region isn't well-conditioned, then DO NOT attempt this exercise. If done correctly, you will feel it work. Be sure to work both sides. This isn't a pick-you-favorite-side routine.
Ventral: the basic idea is to support all of your body weight on the small of your back. Keep your straight & legs together, & lift them up, but focus on your stomach muscles & avoid using your hip flexors. "Push" your arms straight & forward over your chest & lift up your upper body so that your torso rolls up & forward. Again, focus on the stomach, use the gluteals top control the lower body & keep you rlimbs straight - & I do mean straight (not overstrained, but in a controlled manner). Imagine doing V-ups, only you don't actually lift up & touch your toes. Rather, you initiate the movement, using your stomach to lift both ends of the body. Once you've "rolled" your chest forward & up, & your ass tight, legs straght & up, & toes off the ground, hold the position, focusing on the stomach muscles for lifting & the other muscles in your limbs & torso to hold everything together. If done right, someone should be able to grab you by the feet & spin you around w/ only the small of your back contacting the floor (assuming the surface is slick enough).
Dorsal: Rolling over onto your stomach, stretching your arms straght in front of you & keeping your legs straight, use your back & stomach muscles to arch up so that only your stomach touches the ground. Keep your legs & arms straight the whole time. Again, your glutes, legs, arms, etc. should all be working together so the body is a solid unit. This will focus on your back as well as other muscles (There's actually no such thing as isolation workouts, despite what "pros" say). If done right, someone should be able to grab you by the feet & spin you around w/ only your stomach contacting the floor (again, assuming the surface is slick enough).
We called these "log rolls", & would hold position on one side, then roll over & hold it for the other side. Try to work up to holding each position for ~10 sec. Since it's a static exercise, it won't put you at the kind of risk that dynamic lifting does. That doesn't mean, however, that it's cake.
Caveat: I'm no MD. Be sure to consult your physician 1st before performing any exercises. Engage at your own risk.