Tip the chair back a little before you seat the patient. Sometimes having the doctor do the exam first (because some patients will allow the doctor to lay them back but not the hygienist), then don’t sit them up when the doctor is done and you sit down immediately to do treatment may help get them back a bit more, even if just for a short while. I know, sounds kind of like your tricking them, but if it works… Otherwise, standing (like Taxymom suggested) is a great option, along with having them tip their head as much as they can to help accommodate you. Just do your best with what you have to work with.