Two things have worked for me. (a) Switching hands. (b) Trying to use the mouse and the keyboard without constantly forcing the hand into a horizontal positio

Two things have worked for me. (a) Switching hands. (b) Trying to use the mouse and the keyboard without constantly forcing the hand into a horizontal position (right hand thumb pointing left). Instead I got into a habit of holding the mouse in a more (not completely) vertical position (thumb up) and pressing the mouse button with the side of my finger.