I was HSE coordinator for 3 years, and RSI (which is what this sounds like) was my pet specialty. The most effective way to combat it is to take FREQUENT breaks (at most every 7 minutes). The breaks themselves need not be longer than 15 seconds, during which the hands should come off the instrument and simple palm-stretching exercises carried out, or rubbing of the wrists and the areas where the straps press.

And at least every hour there should be a longer break (10 minutes), during which neck-stretches and general arm- and back-stretches should be done, to encourage good circulation.