Countless crunches and multiple sets of Russian twists might tone your abs, but they're not so great for your back. Not only that, but they aren't functional movement, meaning the strength gained in these exercises won't really help you outside the gym. Repeatedly rounding the spine in a crunch or full sit-up sets the stage for a disk injury. Repeatedly twisting, especially if your abs are weak, also puts stress on the spine.
The safest and most effective way to work your abs is to use them to stabilize your torso against motions. The elbow plank is the perfect beginner ab exercise, since it works the extensor muscles, which line and stabilize the spine, in combination with the abs. Adding leg lifts to your plank, while keeping the pelvis and ribs stable, will take the plank to the next level. Working with free weights, moving from a squat into an overhead press, will work your abs too. Work one-armed to force your obliques to kick into action.