A rush for a seat is unnecessary because most shuttles are designed that everyone has a good view going and coming back. You typically keep the same seat as you get off and on the bus at various stops. Most importantly, pay attention to the time and when you are supposed to get back on the bus. It is annoying to keep an entire bus waiting because you decided to buy something at the last minute that you looked at an hour ago. Tip the guide ... extra if they are really good!
Also, at some stops the buses do not always stay so before leaving anything on the bus ask your guide and or driver first. Also it is good to know if the bus will be staying in case you decide to return to it a bit early.
Also, in places where there are other shuttles that look like yours---make note of the bus number or letters if they are present. Even try to remember what the driver looks like.