one other possibility is that perhaps some core work would help also. Your strong core represents a strong center against which all the muscles in your back, rump, arms and legs brace and pull so anything you can do to strengthen and stabilize your core should help. You might also want to doo some arm and shoulder strengthening so that you aren't sinking into your back or letting your shoulders ride up into your ears.

As for stretching beforehand, a recent article in either roadbikereader.com or bicycling.com would seem to indicate that stretching before you ride is not as optimal as stretching after and giving yourself plenty of time of easy riiding and chicken footing to get yourself warmed up.

Just my thoughts.

marni