Don't wait until it's too late. Chair arm rests are optional. If used, they should be padded, not hard. Wrist rests are sometimes recommended but may do more harm than good. If used, they should be well padded and the proper thickness.
Short rests taken often provide better protection than longer breaks after longer periods of work.
The best treatment for RSI, of course, is prevention by proper arrangement of your computer workstation and reasonable job or project design. But if you have even mild symptoms, don't wait to take action. Contact your primary care provider for an evaluation. If you do not have a regular doctor or nurse practitioner in the MIT Medical Department, you should make an appointment with the next available clinician. If you are on a non-MIT health plan, make an appointment with your primary care provider under that plan for an initial evaluation.