get drop bars, so then you can alternate your riding position from time to time, i find that helps.
Having a stem thats too long is bad for your back, but also having it set too low also is bad for your back.
However in my opinion the best thing is the drop bars, alternating your position mid ride really helps a great deal.
You have the top bar position, the brake hood position, the side of the brake hood position, the ends of the top bar, then you have the various positions using the lower bar.
this allows you to stretch your back in ways a flat bar won't allow you to do unless you sit up and let go for a while.
If your worried about braking whilst in the top bar position like i was, get crosstop levers which are available by a few manufactures, like for instance Cane Creek.
get drop bars, so then you can alternate your riding position from time to time, i find that helps.
Having a stem thats too long is bad for your back, but also having it set too low also is bad for your back.
However in my opinion the best thing is the drop bars, alternating your position mid ride really helps a great deal.
You have the top bar position, the brake hood position, the side of the brake hood position, the ends of the top bar, then you have the various positions using the lower bar.
this allows you to stretch your back in ways a flat bar won't allow you to do unless you sit up and let go for a while.
If your worried about braking whilst in the top bar position like i was, get crosstop levers which are available by a few manufactures, like for instance Cane Creek.
hope that helps
dylan
P.S.
Posture is so important and sheldon here ---> http://www.sheldonbrown.com/pain.html#posture
goes into detail about what's what
and about cycling and pain in general here --> http://www.sheldonbrown.com/pain.html