As far as camera settings:
When shooting people, generally you'll want to keep your shutter speed no slower than about 1/60 for someone who's good at posing, 1/125 or faster for average people.
If you have plenty of light: Keep your ISO at 100, pick your aperture for your desired creative DOF, and choose a shutter speed that gets you the proper exposure. If there's too much light, you may hit the max shutter speed for the camera body (1/8000?) and need to either close down your aperture or use a neutral-density filter.
If you're working with insufficient light, you'll first set the aperture you want for your desired creative DOF (maybe wide open), set the shutter speed as low as you feel comfortable while still being able to freeze motion (1/60 for a good posed, 1/125 to play it safe), then set the ISO for a proper exposure. If you start to reach the upper bounds of your comfort zone in ISO, you can either open up the aperture a little more, or find more/better light.
A little sensor noise from ISO is almost always preferable to motion blur. Image Stabilization will help reduce blur from camera shake, but it won't stop blur from the subject moving.
Always focus on the face/eyes/eyelashes unless you have a very good reason to do otherwise. Missed focus will kill an otherwise good portrait.
When framing and composing, don't forget to look past the subject at the background. If there's clutter in the background, recompose or move. If junk in the background detracts from the subject, then the portrait is not a success.
If you have room to work, stick to the 70-200, and stand somewhat far away to minimize unwanted perspective distortion. Shooting up close with the wide end of the 24-70 will introduce perspective distortion, which can be very unflattering if not controlled properly.