When shooting portraits if you get too close to your subject you'll exaggerate body parts such as the nose which isn't flattering. You'll also be in your subject's personal space which can make some people feel uncomfortable and as a result, they won't photograph well. So, what's the solution? Well you can move further back but if you go too far back not only will you need a megaphone to direct your model you'll also end up with featureless portraits. Instead, if you're shooting tighter framed portraits such as head or head and shoulder shots, focal lengths of 85mm-135mm are popular choices but a 70-200mm zoom lens will work too. If you need a shot that includes the background or want to shoot a full length portrait a lens such as a 24-105mm would work well.
Longer focal lengths produce a more pleasant and natural looking portrait which have a pleasing perspective and good bokeh. The blurred background which you get from using a longer focal length or/and a wider aperture isolate the model and make them the focus of your image. The compression the longer lenses offer, particularly with a wide to moderate aperture, gives flattering features which is something all models want.
If you want to work hand-held with longer lenses make sure your shutter speed isn't slower than your focal length. What we mean by this is if your focal length is 150mm don't let your shutter speed drop below 1/50sec. Others believe your shutter speed should be at least double the focal length you're shooting at.