A couple of tips. The first is very important: give the model/subject a brief introduction to the way in which the start of the shoot is going to be handled. In other words, explain "the method" that will be used. Let's say the model is a young female; so we will call her "her". Tell her that you will shoot two or three frames, then you will tell her how to refine the pose, and that you'll shoot three or four more frames, and then we will move on to the next variation of that pose, or the next pose entirely. And then hit these critical points: 1) not every frame needs to be a big smile. 2) We want to capture a variety of expressions 3) We're looking for one out of every ten or fifteen shots to be a keeper, so don't worry about making every shot "a keeper". 4) Closed-mouth expressions, quiet expressions, looking at the lens, looking above the lens, looking off to the side of the lens, or looking wayyyy off to the side--any and all of those "eye directions" can potentially be good, depending on the shot and pose.
The thing is, a lot of people want to please; they want to "do well", but they are NOT really models, so they need to be told, in advance, that not every shot needs to be a big smile, not every frame is expected to be perfect, and that ANY expression might prove to be appropriate.
After a few minutes, new subjects warm up and get into a groove. Give them some positive feedback after about two minutes of shooting. "That was great! That looked good!" Or, "Yes, when you look off to the side from there, it looks good, let's try a few where you look lower, more toward the ground, but keep your chin angled up about the same as the last pose," and so on. After 10 minutes, tell them, "Let's take a break, let me show you how great some of these poses you did look!"
Then scroll thru with her looking on, and when you see a good one, zoom in on it, and give it a compliment, then zoom right thru 10,15 more, until you hit the next "keeper",and praise that.
This approach works. It builds rapport, gives feedback,.and builds subject confidence in the way YOU shoot a set. This is a form of what is called pre-selling, by building enthusiasm for the product, which is the photographs you are making. You are using the d-slr and its built-in review as a tool to help things go better. You allow the subject to literally SEE how well she is doing, and how her "looks" are being seen by the camera and lighting, and she can literally see how well you can run the camera to actually make her look good. Once the subject is in a good groove, do not stop to review again until the end of the first clothing change. Shoot,shoot,shoot.
At the 30-minute mark, you might hit peak stride and get the best shots of the session. Be prepared to shoot that intensely, and without interruptions of breaks.