That's a wide-open question, but here's what I can offer in way of advice: for starters, make sure the model is comfortable. You'll both probably be nervous when starting out, and if you both dwell on that and you get wrapped up in the camera, it'll show in the photos. Better to keep up a running conversation; say anything, talk about stupid things, get the model to engage and laugh. You'll both relax and the shoot will go better.
Building on that, shoot at unexpected times. The whole "one, two, three, cheese!" thing doesn't work well - you end up with forced photo grins. Shoot on two instead of three if you have to count, or just shoot randomly in the middle of a conversation - try to capture natural expressions, like when someone laughs, or makes a face, or smiles naturally.
Finally, don't be afraid to move about and experiment. Don't shoot everything with both of you standing - get the model to sit on something, or lay down, etc. And do the same yourself - shoot from above, from below, shoot through things. Work one side of a pose and then switch to the other side, just to see if it works. Better to take a ton of photos than too few. Once you're done, review them carefully and make some notes on what worked and what didn't for next time.
Good luck!