What are some good ways to avoid adding the 10 pounds to people when you are taking portrait photos?

Very interesting question indeed ;) and applicable to every portrait you take, whether with the nicest model on earth, your beloved one, or mother-in-law.
For the later, I wonder if I want – or not – to add 10 pounds to her face.
Sorry … As a matter of fact, not adding 10 pounds to people when taking portraits is a contribution of the four parties involved: the model, the photographer, the camera and the light.
Let’s start with some recommendations for the model.
No need to look like one of these top ones who appears on glossy magazines.
By the way, don’t be intimidated by the camera, most of these super good-looking ones actually look like you and me 23 hours a day; they just look perfect once made-up and photoshoped.
Ideally, the model may want to (1) turn her shoulders sideways to the camera, (2) pull her head forward and (3) pull her shoulders backwards.
Not to the extent of looking like a disarticulated puppet, but this kind of poses tends to give a thinner look.
Shoulders sideways will minimize the visible ‘surface’, thus looking thinner.
Head forward may hide a bit of double chin if any.
And shoulders pulled backwards give a more dynamic look.
An experienced photographer would help the model posing the best way.
Next post to address the photographer’s contribution (including camera angles and light).
Bernard

The pose itself is very important as the other answers suggest, but also keep in mind that if you are the one who is clicking, the type of sensor on the camera and the lens used itself make a big difference.
Try using a longer, prime lens (Shorter lenses tend to get wider images) to take portrait shots, and there would be lesser distortion to the final image than on a cropped sensor camera with any other lens.
There are conversion charts available online which anyone can use for specific lenses and camera models that would make your shooting more effective and efficient.
Hope this helps!

Taking side shots as opposed to head-on shots.

This ways have 5 secret answer and tell thay are now in here Secret No.
1: Turn partially sideways to the camera, planting one foot in front of the other.
Point your toe to the camera and place your weight on your back foot.
Secret No.
2: Pull head forward slightly to minimize any appearance of a double chin.
Secret No.
3: Hold arms slightly away from your body.
This keeps upper arm flab from flattening out and therefore appearing flabbier (much like thighs do when one sits on a couch).
Secret No.
4: Pull shoulders back, chest forward and gently suck stomach in.
Be careful not to suck stomach so far in that your ribs show, thereby causing those who later see the photo to cluck to themselves in a bemused, sing-song voice, "She's sucking i-in." Secret No.
5: If you can get away with it without looking like a Sports Illustrated Swimsuit Cover Girl wannabe, try the look away trick.
To do this, look away from the camera, then turn towards it, breaking into a smile just before the camera clicks.
Your smile will appear fresh, not frozen.
This trick takes practice behind closed, locked doors.