In a portrait, a distracting background detracts from your main subject. If you have a DSLR and a lens with a large aperture, you can easily dcrease the depth of field to blur the background. Just use a large aperture and see how nicely the background blurs and isolate your main subject, putting the focus right onto it.
However, if you are forced to use a small aperture or you have a point-and-shoot camera and the depth of field is too great to blur the background, you can still have recourse to Photoshop to blur the background. Of course, this is now digital manipulation and you should indicate this as thus.
DPNow has a nice tutorial on it though it does not use the latest edge detection feature of Photoshop CS5.
Read the tutorial at: DPNow.
Read our tutorial using Photoshop Elements: Isolate Your Main Subject. It uses radial blur for a more fantastic effect but you can use a less obvious blur for a more realistic effect.
Read our tutorial on Bokeh (the real type).