Current thinking is you must build up your 29 core muscles (area from the knees to the nipples) not just abdominals.

Old Philosophy Sit-ups - excessive compression of spine Posterior pelvic tilt - increases risk of injury Flattening the lumbar region

New Philosophy

• Muscle endurance vs muscle strength
• Maintain a neutral spine
• Flossing - active range of motion
• No distinction between - upper and lower abs

- Curl-ups for rectus abdominis:
- Several variations of the side bridge for the obliques, transverse obdominis, and quadratus:
- Leg and Arm Extensions leading to progressions of the bird dog: for many back extensors

Advanced Exerrcises

- Back Bridge
- Cat Arch or Camel
- Side Leg Raise Can do with one or both legs
- Swiss Ball Exercises Curl-up Extension Bird Dog Back/Shoulder Extension
Routine
Static exercises should be held for 7-8 seconds,with 3- 4 seconds rest.
Mad Cat Curl-ups Side Bridge Front Bridge Extensions	5 - 6 cycles 5 - 25 reps 3 - 8 reps 3 - 8 reps 3 - 8 reps

Source: Core Training at Hennepin County, MN

Single Knee Pull: to stretch lower back and back of leg. Lie on back, hands at sides. Pull one leg to chest, grasp with both arms and hold for five counts. Repeat with opposite leg. Suggested repetitions: 3 - 5.

Double Knee Pull: to stretch lower back and buttocks. Lie on back, hands at sides. Pull legs to chest, lock arms around legs, pull buttocks slightly off ground. Hold for 10 to 15 counts. Suggested repetitions: 3 - 5.

Seated Pike Stretch: to stretch lower back and hamstrings. Sit on floor, with legs forward, knees together. Exhale and stretch forward, slowly sliding hands down to ankles. Stretch only as far as is comfortable and use your hands for support. Hold for 5 to 8 counts. Don't bounce, position inhaling deeply. repetitions: 3-4.

See: President's Council on Physical Fitness and Sports for more.

Supine Spinal Twist (Supta Matsyendrasana)
 
See Spinal Twist at westvalley.edu for instructions.
Other Stretch Excercises at https://www.westvalley.edu/stretch/

Your body's core is the area around your trunk and pelvis. When you have good core stability, the muscles in your pelvis, lower back, hips and abdomen work in harmony. Strong core muscles make it easier to do most physical activities - from swinging a golf club to getting a glass off a top shelf or bending down to tie your shoes. Weak core muscles leave you susceptible to poor posture, lower back pain and muscle injuries.
Source: Mayo Clinic

Yoga for Back Pain - Back Pain Relief with Yoga
Yoga for Back Pain - Heal Back and Neck Pain with Yoga
Yoga for Back Problems
Proper Lifting Techniques

Links:
Core Training at Hennepin County, MN
Exercises for a Healthy Back at Canada's National Occupational Health & Safety Resource
The Mayo Clinic: Core Exercises | Back Pain | Core exercises: Beyond your average abs routine
Abdominal and Core Strength and Endurance Exercise List at sportsmedicine.about.com

15-Minute Foam Roller Tutorial for Low Back Pain Relief | Ashton August, YogiApproved.com
TriggerPoint Roller at Amazon $26

  See also Amazon.com : SIUSUMFO Vibrating Foam Roller, High Density Deep Tissue Massager with 5 Speed of Vibration for Physical Therapy, Exercise, Muscle Recovery & Performance : Sports & Outdoors $75

Cure Back Pain Permanently Without Surgery - BackForever Program (Katy recommendation)
Exercises to prevent back pain at orthop.washington.edu
Train Your Abs the Right Way!
WebMD
Strengthen your core at about.com
Stretching:
President's Council on Physical Fitness and Sports