Have you ever noticed that the first 30 minutes of a shoot is a waste of time? Everyone involved is getting comfortable with the location, the sound of your voice, a feel for the directing and actually, in a subconscious way, the mood is being set for the rest of the shoot.
If it's a model it's a good idea to find out how long they have been modeling. And what they plan on getting out of the shoot. For example, if they want pictures to submit to an agency, don't ask them to pose in lingerie the whole time. Those are self esteem shots only. Unless it's a male model and I don't EVEN want to hear about that.
In a trade shoot the communications has to be there to make sure everyone walks away with something to show for the effort. As important, if someone is new at modeling, give them all the advice you can about posing. In a positive manner talk with them about angles, which ones are best for them, how to think through poses between flashes, how to hold their chin out, and how it's not personal when we point out that wrinkle on their side and have them stretch to take it out. (grin) If they walk away with some great pictures AND some advice and tips to consider to make their modeling just a little bit more focused and refined, it was a great shoot.