Make sure you dress up nice, that way they feel impressed. .
Honestly though, phone interviews are a cinch. You should be less nervous about this than an in person interview since you don't need to worry about presenting yourself physically. Just make sure you are well spoken, and don't lie. If they ask you a question you don't know the answer to, tell them straight up. You don't want to tell them you know x, y, and z, and then when you get the job, you are fvcked,
For my current job my boss asked me on the phone if I knew Java, and I told him no. My job is very heavily dependent on Java, and if I told him I knew it, he wouldn't have given me the first few weeks to get settled in and learn Java. I have a very impressive resume, so not knowing Java was fine, since I am a fast learner.