In reply to MarkRoe: Are you aware of why you tense up your shoulders? Good form should leave your upper body relaxed, so I'd suggest some form focused sessions.
The two regions that can help this are short and very fast reps, and long very easy ones.
For the former the easiest is to look at strides or 200-400m reps off pretty much full recovery. Try to consciously relax as you accelerate to just less than sprint pace, looking ahead and keeping the turn over high. It's hard to run very fast whilst tense. If you still find it hard there are some specific drills, but I think I'd fail to describe them.
The other is just to go out reasonably long, but again consciously relaxed. Don't worry about pace and eventual distance, but from early on try to feel the effort is easy and concentrate on a smooth easy form. If you're form starts falling apart either slow down until it becomes easy again, or stop the session there.
It can take a little time, but you can waste so much energy if tense it's worth working on an easy form.