i've had bad backs for years, and took up yoga after our son was born six years ago as lifting him then nearly did for me. i do it every day for 5mins in the morning at least, and my back, despite the odd twinges, has never been so good for so long.
famous last words. but I fully support yoga as a great exercise.
there is something called the cobra for bad backs, which is worth checking out. you can also lie flat on floor tummy down, doingnothing. very relaxing,
Check out a dorling kindersely book on yoga, great stuff, it all gets easier the more you do it.
and sit up straight!!!