Friends ask me what lens he should get for portraits. In normal situations, you want to get the sharpest image as possible. Also, female models will complain if you make them looks fat. Under those conditions, you would like to get a sharp lens with at least 50mm or above (longer focal length can make models looks skinny :p ).
If you have a Canon camera, I suggest to get a Canon EF 85mm f/1.8 USM for around $400.00. It is a very sharp lens and didn't cost you much for taking good photos of pretty models.
Click "here" for the full size image. You can see her in detail and find out how sharp the lens is.
If you are Nikon user, Nikon 85mm f/1.8D AF Nikkor Lens maybe what you should look for.
Message from a Nikon Pro: Nikon 105mm f/2.8G ED-IF AF-S VR Micro or Nikon 105mm f/2.0D AF DC should be better for Portraits.