You lie on your back with knees forming a pyramid and hands behind your head. Slowly lift your shoulderblades off the ground twisting alternatively to the left and right and slowly recline. Do it at your normal breathing rate - breathe out while raising and in while going back. It's a lot harder than it sounds but if you're in halfway decent shape you should be up to 100 within a couple of days.
This takes a fair amount of self discipline. Frankly, I haven't done it much in the past few years but bicycling, for some reason, doesn't bother my back.