Make good friends with your "heart pillow" Coughing without it *hurts*.

Make sure your anesthetist knows you have MG - there's an order to the cocktail they give you to put you out if you get it in the wrong order it is quite unpleasant. There are muscle relaxants included in the cocktail, if they know you have MG they'll send those in after your out rather than before (I was put out for a different procedure before my thymectomy and they had the order reversed... felt like I was drowning.)

If you're hooked up to a patient controlled pain management system, don't wait until it hurts before you press the button. Keep your pain as well managed as possible during post-op.

Follow the orders regarding lifting/moving stuff, you don't want to knock your sternum out of alignment while it is stitching.

Get a wedge or something similar... you'll want to sleep propped up at least partially for a while when you're healing.