Tamar,

>Get a chair with armrests. Keep your elbows on the armrests. That's step one. Your arms shouldn't be hanging in the air.
>
>Step 2 is keeping yourself from bending your wrist heavily to use the mouse. A wrist rest in front of the mouse pad (or as part of it) helps a lot. My keyboard arm has it built in. It keeps me from sitting with my hand on the mouse and my wrist bent at 45 or 60 degrees while I'm reading.

Thanks for the tips.