When I feel like I'm tensing up, I envision my arms being like limp linguine - I flap my elbows in & out, roll my head a little & that helps my form. I draw my shoulder blades back & down to keep from scrunching my traps up to my ears.

Make sure you have a slight contraction to the lower abs to help stabilize your lower back/core.

Having an experienced cyclist at your LBS fit you certainly can't hurt.Experienced - that's the key.