AS far as being invited aboard as crew, wear hot weather clothes, boat shoes with soft white soles and carry foulie rain gear. Keep a boat knife in your pocket, fingerless gloves help soft hands survive. If you bring anything aboard bring it in soft bags that are easily jammed into lockers, not hard sided coolers or suitcases.
Etiquette - handle binoculars and winch handles extremely carefully. NEVER set them down on deck. Always set them back where you found them. Nothing ends a nice day like dropping a $500 set of binoculars over the side.
Volunteer for every dirty job. Fix sandwiches, swab decks, coil lines, haul up the anchor, etc. I used to help paint the boats bottom every spring. That guaranteed me a couple of days of sailing. Buy lunch. Tell the skipper that he made the boat payment so you can buy lunch. Guarantees a re-invite every time.
Be careful with actual sailing things, let the skipper tell you how he likes things done, even though you may know how. Every skipper does the same things differentlty.
Have a good time.