+1 to Marni. Core strengthening can help back issues so much. Having a stronger core will make you a stronger more stable cyclist all around too.

Fitness/yoga balls are great and are a pretty cheap way to do core exercises - and balance exercises as well.

Another thing - do you have health insurance? - many insurers cover a bike fit by a PT these days. You may be able to have a good fit done for little more than a co-pay.