go in with an open mind
Ask in advance what they want you to do
Ask what they will allow you to do
ask many many questions
make sure you help them with cleanup and changing cots
DO NOT spend most of your time flirting with the nurses, talking or texting or playing games on yoru phone. Youare there to learn and not to text or talk on your phone. If I as a FTO/evaluator had my wish and ruled the world, I would not students to carry their cell phones on them when they were doing ride alongs or I would put them in the lock boxes and I would only allow them out during breaks and lunch.
Bring your field guide or some book to let the medic know you are serious about learning.
Do not offer to buy them lunch or dinner, that would set a bad precedent that you would be held to on your next ride and your next and again and again. You are there to learn, not to buy the crew food.
I'm sure that there will be other stuff that others will add.