Isometric Back Extension. No Roman Chair or Back Extension machine, no problem! Grab a swiss ball and find a sturdy place low enough to securely anchor your heels underneath. The ball should be at the level of your pelvis, the higher up near the belly, the less effective the exercise. Keeping the spine in neutral, bring the torso parallel to the ground by flexing at the hip as necessary, and hold for time. Breathe slowly with control. You'll inevitably feel the lumbar erectors in your lower back burning for oxygen. I usually prescribe 1 minute holds for a set of 3-5. Isometric training for the core is the best and safest way to train, and has been demonstrated to translate into dynamic stability, strengthen and performance as well.
Progressions are shown here, for those new to lower back exercises, begin with the arms crossed over chest. Once these become easy to perform 4 sets of a minute holds, try advancing to hold hands clasped in front of face. When this position is mastered, advance to hands over head. A 5-10lbs plate or dumbbell can also be held at any of these positions to increase difficulty before progressing to the next position.
Some common mistakes: ball under the belly (too high) or under the thigh (too low), torso not parallel to the ground, and lumbar spine not in lordosis. Just be bright and do them right