Hopefully the fit will help a lot.

To expand on something Dogmama said, though... I think she's right that it's really common to ride with tense shoulders. In addition to the stretches she recommends, I find it useful to remind myself often to keep my shoulders down. To illustrate how they should feel, stand over your bike and hold your bars as if you were riding. Quite possibly, you will find that you are "holding yourself up" with your shoulders such that your shoulder blades raise and come towards each other in back. Now, standing in the same position, bent over at the waist, let go of your bars and let your arms hang down, really letting them go, letting gravity pull your hands towards the ground. Notice how different your shoulders probably feel -- notice where your sholder blades are -- probably much farther apart, right? This is how your shoulders should feel when you ride. At first this may be a little tiring to your core muscles, but that will change quickly.

Every couple of miles I check in on where my shoulders are -- sometimes, even if I think they are down, I will try to push them down as far as I can, and then relax and let them come up a little, just to make sure I"m not holding tension there. It really helps, I don't ever have neck/shoulder tightness now.