I have this problem frequently enough that I have a whole itinerary of stretches.

First stretch:
1. Tilt head to right so ear touches shoulder
2. Pivot head so you're looking down toward your left pectoral
3. Raise your right arm and rest your palm above your left ear
4. Pull with your right arm and hold for a 10 count
5. Repeat 1-4 with other side
-- You should feel the stretch around the left side of your trapezius and it should work out the blue, green, and red muscles in this diagram