This is entirely dependent on the model. Some I'm dead serious with, others I flirt with, some I tease, with some I'm self-deprecating, with others I do my best Austin Powers impression... The list goes on.
Mostly I read the model and find what works for them and that is usually a combination of all of the above. First you need to understand what motivates them, and then you need to adjust your demeanor to be able to achieve that. I find humor is usually important when dealing with new models as is being firmly in control. This does not mean that you are a drill seargent on set, what it does mean is that you have your stuff firmly together, are well rehearsed and that you know exactly what you want to achieve and how you plan to achieve it. If there is a leadership vacuum, a more experienced model will take charge. You need to establish very quickly that this is your shoot and that it will be run by you. Now, you could do this by being bossy, but that rarely works. Instead, just lead by example. Smile, be fun, but keep everyone working on time and on budget. This is one of the reasons I spend a lot of time in preproduction. Once we hit a location or a model arrives at my studio, it's go time. I'm shooting a model tomorrow (hopefully, her son is a bit sick) I've spent days building the set, testing the initial lighting plan, making notes of how I want her posed, purchased wardrobe and accessories, etc. I'm ready to go. If I have crew on a shoot, they are ready to go. As soon as the model walks through the door, it's a big smile, a euro "air" kiss to the cheek and then she's off to hair and makeup. As soon as she steps on set I'm ready to put her in position and begin metering and tweaking the lighting plan. While I'm doing this, I make small talk to get to know the model, if I already know her, we chat, I tell some corny joke I heard recently or answer her questions regarding my non-existant love life. But all the while I'm working, smiling, but working. I'll remind her of previous conversations regarding what I'm going for in that shoot and compliment her. Once the camera is up I take a few test shots, and check them out on the computer. While doing this I keep up the chatter, always talking, and then we discuss the tests, I make adjustments, test again and then we shoot. By that time, if I selected the right model, it just flows and I get what I want. If not, then I have to try and figure out how to get it.