What do the rest of you do to avoid back problems. From crouching/leaning over my model for extended periods of time, my back is killing me!!

Professionally speaking (sort of) the only advice I can offer is to use an adjustable chair - height and back. Correct posture is the key as its understandable that back probs should occur if leaning over for prolonged periods of time so take regular breaks, even if no pain is present.

I'm 100% with Hip_Priest on this. I suffer with my back since I had two accidents one on a tank and another one on a helicopter. A good chair helping to keep the right posture is essential. I can work on my models for hours without any problem...
Ciao

Exercise. I run between 15-20 miles a week and normally do between 90-120 push ups and sit ups daily (depending on lifting/run days) and work a universal weight gym at work. Helps keep you overall physically fit and maintain proper posture.

I totally agree with the above statement on posture and exercise. I used to have the same problem until I got one of those little turntables from a craft store. Found it the cake decorating section.
What this does is elevete the model off the bence just enough so it is the right height so no leaning is necessary plus you can work on any side of the model w/o having to pick the model up. Just turn it to the angle you need and work away. HTH