There is a lot of articles out there on how to build core strength to prevent back pain. For me the vast majority needs to be working on core stabilization vs just strength. I have seen a ton of guys at the gym that do massive amounts of situps and crunches but still complain about back pain.
The problem is 3-fold.
- The core is made up of a lot more than just the 6-pack (rectus abdominus). There is the obliques and transverse ab muscles, the low back extenders (QL and paraspinals) and the gluteal muscles. They are addressing only 1.
- They are working the muscle primarily for strength when shortening the muscle (concentric) while most low back injury prevention needs lengthening control and strength (eccentric contractions).
- The are seeing how much the muscle can move rather than how much stress on the muscle can you have without moving the spine out of optimal position.
Planks are a great exercise. Side planks will get the sides of the abs as well. I do like ball knee tucks. Because you are further from the 2 points of contact, you get good eccentric work bringing the knees back. You are directly in a plank position and can work that in with the workout.
Another great exercise is the spider pushup. For those that are advanced you can 
do that from the same position on the ball.
Finally, Single leg dead lifts are a great exercise. Please make sure to hip hinge and see the previous post on it.
