I have a tendancy to let my shoulders climb into my ears so I constantly remind myself to push my shoulders down and back. Core exercises especially planks seem to help as do pushups, but it wouldn't hurt to check the angle of the handlbars and seat- but basically, most of us tend to be round shouldered with upper soulder and upper back muscles weaker than they should be.

you might also try doing some neck and head rolls each time you stop and step down.

good luck

marni