Hey, I guess all basic advice is already covered now.
I could add not to hinder the service, e.g. help pass the food (if any...) trays.
Also study the mechanism of the seat harness buckle. Most airliners have side belts/scrotch belt/shoulder belts. When you turn the release handle, it frees all ends (but for the one it is attached to). To feel more confortable, during climb or cruise, most pilots release only the shoulder belts. This can be achieved by pushing a release tab (or lever) located on the top of the buckle, just behind the shoulder belts inserts.
Have fun and enjoy
