i was told that your stretch only becomes "useful" when you hold it for more than 25-30 seconds. as such, i'd suggest one minute on each stretch.
also remember to strengthen your core.
planks (facing down, side planks, facing down with your feet on a medicine ball, and alternating feet planks). as was suggested to me by another poster, the superman maneuver helps, too (laying down, arms stretched, lift both arms and leg up, hold for 2 seconds, release, repeat)
six months ago, i had a +6 stem with three spacers underneath, im now down to -6 with no spacers. next item is a -17. although the bad thing to all this is that i now need to tilt my saddle down a bit as the boys get crushed a bit... getting progressively more aero is the key.