I have recently worked with a couple of massage therapists with thumb injuries. I have also worked occasionally in the past with guitarists with a similar arthritic symptoms of their thumbs.
In all these cases there is a common thread - the hyper-extension of the thumb whilst playing or massaging. Such hyperextension is a common cause of arthritis in other joints of the body also - the knees being a prime example. Basically if joints are constantly pushed beyond their resting range of motion pain is likely to occur.
So part of the answer is to always use the thumb with a slight flexion in the interphalangeal joint and metacarpal-phalangeal joints. Also if you are overextending through these joints it is also possible that you are overextending in the elbow joint and probable that you are jaming up your glenhumeral joints and whole shoulder girdle and at the same time hyperextending your neck, putting you in danger of further mucsculoskelteal pian and also nerve damange which may refer through into the arm and hand.