Avoid toe touches, double leg lifts and full sit-ups.
Go for partial crunches, bridges, wall sits, bird dogs, press-up back extensions, knee to chest, pelvic tilts, and hamstring stretches.
Also, personal trainers have told me to avoid the back extensions weight machines at the gym. Because you are supported by the machine, it gives the illusion that you can do more weight than is really advisable. It is almost always more effective to do free exercises that recruit multiple muscle groups to stabilize you even while they focus on certain muscles.
Massage therapy is also a proven aid to any training program or recovery from strain or injury.
To see a slide show of good and not so good back exercises, follow this link: