To stretch your chest, bring your arms close together behind your back. You can clasp hands, although this will impede a maximum stretch. I used to do this with several clients: A personal trainer or knowledgeable friend can come behind you, place their knee against your back (not on the spine) and bring your shoulders and arms back as you exhale while moving your chest/ribs forward with their knee and weight. Needless to say, this should be done with care.
To stretch your back, hug yourself, your hands wrapping around the opposite side of your rib cage as soon as possible. Hold for 10 seconds.
Disclaimer: None of the above information can be taken as a substitute for advice from a medical professional such as a physician.
My third book, Pocket Guide to Fitness, is available on www.louizapatsis.com, http://www.authorhouse.com, www.bn.com and http://www.amazon.com. If you look up my name on those Web sites, you will find my other books The Boy in a Wheelchair and Life, Work and Play: Poems and Short Stories.