A commonly tight muscle in lots of people who spend the day sitting and leaning forward onto your handle bars can make this worse.

Stand tall and take your arms down and out to the sides.  Gently take your arms further behind you and slide your shoulders away from your ears.  Take care not to arch your back.   Repeat but this time with your hands by your ears to stretch across all the muscle, take care not to pull your head forward.  You can also stretch your pecs by placing your hands either side of the door frame and then lean through the door.