I have a rather simple method: play with the model! LOL I'll have someone cracking jokes behind me while I shoot, or if its just me and the model I'll be the one joking. Let's face it, models are models, not actors. I have to deal with women and men that have the "look" I need, but all too often zero acting skills. Because of this, I find it extremely helpful to make them comfy and then purposefully stimulate the model emotionally on set. Every reaction is now genuine. Works great for positive emotions. I've been taking this to a new level this month, and the results are looking great, can't wait upload these babies!
One thing I explain right off the bat to models now is how important their eyes are. Anyone can smile with their mouth, but a genuine smile uses the eyes and forehead. Nailing real looking genuine emotions will get you noticed by buyers - weather it's happy, tired, or whatever else emotion you are going for.