LISTEN TO THE WISDOM OF A FOOL. STRETCHING CAN SAVE YOUR CYCLING SEASON, ESPECIALLY AS YOU GET A LITTLE OLDER.
A few good stretches here:
http://www.cyclefit.co.uk/stretching.htm
It's important to stretch the hamstrings, which can get tight and contribute to a bad back (take it from one who knows). Also if you sit at a desk all day and also spend time sitting on a bike, make sure you do some backward bending to compensate for all that forward leaning (I like to lie back on an exercise ball and stretch out. Very comfy).
It doesn't have to take a long time or be painful. Stretching (yes, along with a higher stem) took my back out of a world of hurt.