In general, for me, slow movements stretching forward then arching back is good. Of course, having stronger stomach muscles helps your back, so I will sometimes tighten my stomach muscles when doing the lean forward part.
Moving your shoulders forward, then back can both relieve tension in your back and chest.
Raising one arm at a time and leaning toward the opposite side and then moving it around seems to help me with shoulder tension as well as my back.
1) stretching everything below your lower back – hip flexors, hamstrings, quads, calves, glutes (all 3 muscle groups).
2)This is best supported by a daily core work out.
3) good form and an understanding of the difference between working a muscle (many people get frustrated with stretching because they are actually doing an isometric exercise, not stretching the muscle and never get more flexibility) and stretching a muscle and associated tendons and ligaments. Relaxation is the key to stretching. As an example: Muscles are there to support your structure among other things, now imagine someone with super tight hamstrings bending forward to perform a stretch and hanging the entire weight of their upper body on their hamstrings, muscles are awesome but they are dumb. This muscle will engage to support the excess weight and stretch and begin to protect. To protect itself and it’s owner the muscle flexes and badaboom badabing you’re exercising and not stretching. It’s best to use supports and find the relaxation in the stretching to get great gains.