I've got a spotty track record of working on strengthening my core. During the winter, I'm usually on a 16 week weight program, and I make really solid gains in strength then. But during the riding season, I neglect my core and ultimately wind up paying a price for it with lower back pain on my rides. This article does a great job at explaining why you might get lower back pain during rides. It does, however, assume that you've been competently fitted to your bike. This winter is the first that I'm doing a mixed riding / weight program. Early signs point to it helping me keep my spine in a neutral position during my rides (losing some weight around my midsection will also help this too!).