As an on-call political analyst, and serial scholarship and program applicant, I've done a lot of Skype interviews, some of which have gone very well, and some that taught me things that might seem basic, but were definitely not basic for me.
Checklist: Level Basic
Take initiative: Learning to expertly navigate virtual interactions can take some time, especially if this is your first time communicating with a possible employer, scholarship interviewer, or admissions committee. Make things easy for yourself, if you are not given the other person's Skype information, give yours and ask for theirs. Be sure to let them know you'll be adding them to your Skype account a couple of days before the interview. This will prevent any last minute back-and-forth e-mails trying to coordinate username exchanges.
Check your audio and video feed: An hour before or during the middle of your interview is not the right time to check if you're easily viewable and hearable.
Practice looking at the camera: This one might sound silly, but in a normal interview you'd be able to focus on your interviewer, so it's important to maintain eye contact on Skype. It's a lot more tempting to focus on what you look like on the screen or to look at the video feed of your interviewer. The real trick to eye contact on Skype is to look at the camera straight ahead. Test it out with a friend! I always call friends a day before my interview to check the audio and video feed and to practice my eye contact.
Dress for success: Dress professionally from head to toe, not from head to waist. As the member of a scholarship committee interview, I had a candidate who wore pajama bottoms. He assumed we would not see them. Unfortunately, the candidate didn't test out the camera angle before the call, and needless to say, the interview got off to an awkward start.
Close other programs in your computer: Having Facebook, Googlechat, or Twitter in the background is distracting, especially if a pop-up window suddenly throws you off in the middle of the interview. Want to take it to the next level? Print your notes instead of reading them from the computer. Your eyes look lower than the camera when you're reading notes from a document in your computer.
Checklist: Level Intermediate
It's all about the visuals: Your space matters. While your initial reaction might be to just worry about finding a quiet space (like your room), a neutral backdrop should also describe your surroundings. Ensure your interviewer only has eyes for you-not a messy stack of papers and clothes, or a collage of posters.
Your profile is your firm handshake: Usually the interviewer is not first introduced to you. Much like you might have heard to keep a professional e-mail, the same applies to a Skype username (with an appropriate profile picture). Feel free to divide your professional from private life now. After all, usernames are free!
Not be afraid to smile. While speaking with your hands can be a little distracting on Skype, a smile can go a long way. Through all the nerves, remember that a Skype interview is still very much like an in-person interview, only with a few more technical bugs and less hand-shaking: smile, you'll make it through.
Checklist: Level Advanced
Expect the best, plan for the worst. If you're comfortable and in the same country as your interviewer, offer a phone number in case technology is not on your side. Better yet, if you're in a different country from your interviewer, ask for a number in case the internet drops. Invest in a little bit of Skype credit, buy a phone-card in advance, and keep both handy if the internet fails.
Internet down! Do not panic. While hosting a Skype interview as part of a scholarship committee, I saw a candidate show poise and graciousness in spite of some technical bugs. When Skype failed in the middle of a key question, the committee called the candidate by phone (which she had provided just in case). She then proceeded to reassure us that it was fine, reminded us of our question, summarized some previous points and continued like a Pro.
Remember, a Skype interview is still an interview. All the typical formalities apply. Prepare your notes and questions; follow-up to thank your interviewer; and get a good night's rest the day before!