I find using a pen really helpful -just as is said elsewhere, it is more ergonomic and extends your reach (I'm also an occupational therapist and this makes sense from an activity analysis point of view); one client actually requested I use a pen as he would find it easier to focus on this than my fingers. Other than this, look at room set up; can you alternate the side of the client's chair you sit so you're using opposite arms? Good luck with your employer, if they're approving the use of EMDR it is a repetitive strain injury that they could be liable for.