Many back injuries can feel quite debilitating, especially to otherwise active people that would still be exercising if not for that specific pain. Without knowing the details of your injury, it's hard to diagnose or provide exact exercises that will help minimize flare-ups, but we can definitely start by strengthening your core muscles.
Too often people view core training as abdominal work only. This will sorely neglect important areas in the obliques and back and possibly result in muscular imbalances that can cause or exacerbate pre-existing injuries. A well-rounded program should include:
Reverse crunches: on your back, curl your knees toward your chest instead of your head toward your feet.
Front bridges: these are sometimes referred to as a plank because you're holding your body in a pushup position with your body straight as a plank from your shoulders to your heels. Work up to holding this position for up to 60 seconds. If this causes any pain, you can modify the exercise by putting your knees on the ground.
Side bridges: Similar to the front bridges, except your body is facing to the side instead of down toward the floor. Your body weight is supported by your right hand or elbow and the side of your right foot, while your body is in a straight line. Repeat on your left side.
Oblique crunches: start by lying on your back with your knees bent and feet on the floor. Now let your knees drop to the right side while your shoulders stay on the ground. Lightly clasp your hands behind your neck and use your core muscles to lift your head and shoulders a few inches off the ground. You should feel this most in the left side of your abdominal muscles. Complete ten to 15 with your knees to the right, and then switch your knees to the left and repeat.
There are countless variations of core exercises that can be done, but these are the basics and are excellent at strengthening your midsection and helping to prevent injury and provide stability to already-injured midsections. Two to three sets of each exercise, containing 12 to 25 repetitions (or 30-60 seconds for the bridges) will give you a great strength workout and only take ten to 15 minutes of your day.