Professionalism: Be the part. You are a model. Believe it, and in yourself.
Etiquette: Simply start with basic human consideration. Be where you say you are going to be, and be on time. If you have to cancel, call as early as possible, before your photographer has shelled out for location costs or wasted time preparing and setting up. Be focused, present, rested and coherent. I once had a shoot with a model who was clearly hung over. In hindsight, I would have preferred that she cancelled.
Input: Collaboration usually comes after knowing someone for a while, or having worked together before. Typically the concept of the shoot should have been planned ahead of time, but there is a lot you contribute by being expressive, natural, and comfortable in your skin. If you are new to modeling, there is a lot that you can learn simply by being focused and attentive, and that can help build a collaborative relationship. I don't mind questions and input, but I imagine this varies by photographer.
Preparedness: Be prepared to take direction. A good photographer should clue you in on what to expect in terms of location, circumstances, art direction and so forth. They should also be willing to share some portfolio work, and/or references, so that if you have any doubts about wanting to move forward you can address them beforehand.
Things that will make your pictures suck:
- If you hate your body.
- If you can't relax.
- If you are self-conscious.
- If you want me to make you look slimmer.
- If you only want me to shoot from certain angles
- If you don't trust your photographer.