From what I am hearing (I counsel with job seekers), Skype interviews are becoming more and more common. They save time and money for both parties, and can often be as viable as a live interview in terms of selecting the right candidate. I know of people who have been hired through Skype interviews.
Here are some tips:
Be sure to look at the webcam on your computer, not at your own thumbnail version of the broadcast. This way, the person interviewing you will feel they are looking into your eyes and seeing your full face.
Smile!!! Just as in a face-to-face interview, they want to see and experience your personality.
Make sure there's no annoying back-light or glare from your end that will distract the interviewer.
Dress as you would for a live interview!!!
Get rid of any distracting clutter behind you or in the line of view of the webcam. This not only detracts from your presence, it might send a message to the interviewer that you're not very organized.
Make certain there are no outside noises - send the kids away for the afternoon, ask your spouse to stay in another room, and make sure the dog and other pets are not near the computer.
Relax and answer truthfully. Being on a webcam is almost like inviting someone into your home, or sitting with them in person. Don't be afraid!
If you're a woman, don't wear too much make-up or colors that will glare on the screen.
Do not wear intricate patterns. These get distorted when viewed on a screen, and will distract the interviewer.
Make your answers brief and to the point. This is the same thing you would do in person!