I also suffer from cracking in my sternum. There is a great feeling of pressure that seems only to be relieved by arching my back backwards to cause the chest bone to crack and release the pressure. I as well have been to many doctors who cannot explain why this occurs.
I believe the syndrome is related to arthritis or joint disease. I also suffer from tmj, a mild form of carpal tunnel as well as clicking and cracking in most of my joints. Since vioxx and other similar drugs have been off the market (they did help) there is not much treatment that seems to work. I can share what I know - please do not attempt any of these remedies without first consulting a doctor.
Ibuproffen 800 mg is effective for acute bouts - mostly when taking at the first sign of discomfort or before. Doctors have advised me that these types of inflammation can go dormant from time to time and are usually brought back on by stress - sometimes simple deep breathing and exhaling breathes done routinely can stave off this symptom.
Icy hot when rubbed on the area can provide some comfort as it seems to decrease the inflammation. Wintergreen oil is also effective.
Seeing a dentist to evaluate your bite can provide tremendous relief as the tempro mandibular joint plays a huge role in joint distress thru out the body. A night guard or bite plate - properly fitted, can work wonders for this type of discomfort and also for back pain.
I have found chriopractors to only provide temp relief. Use lumbar support (an erogonomical designed back cushion) while sitting at a desk for long periods of time. Get up from desk and stretch 10 minutes for every hour - makes a big difference - I first balked at this - but it works.
My greatest relief came from having my bite (alignment of teeth) diagnosed - unusual - but tremendous relief and improvement. An excellent dr for this is Dr. Harold gelb in nyc, but perhaps his practice can recommend someone in your area.
Good luck and let me know if you have any questions I may be able to help with.