First of all, congrats on the phone interview! These calls tend to make people nervous, but since the goal of most phone interviews is to get an in-person interview, just focus on what you would focus on normally in a face-to-face interview, with a few tweaks to allow for the phone element.
Clarify the details: Since you may not be in the same time zone as the interviewer (a common reason for phone interviews), be sure to clarify the time of the interview in both your time zone and the other, and confirm who is calling whom (i.e. "I'll expect your call at 11 AM PT / 2 PM ET. My number is… ").
Use notes to your advantage: The best part about a phone interview is that you can have your notes in front of you (and the interviewer can't see them). So have a copy of your resume, extensive bullet points about the experiences or skills you want to mention, and full list of questions written out ahead of time for use during the interview. You have the gift of invisibility-use it to your advantage!
Dress the part: This is more of a mental tactic than anything else—but dress up a little bit. Looking nice puts you in the right mindset to be professional.
Make sure all systems are go: Make sure you are in a quiet place with a charged phone and a glass of water. Call your mom or a friend beforehand from the room where you plan to have the interview, making sure they can hear you with no distractions. (Note: It might be worth it to run home to take the call from your apartment, rather than trying to find a spot to do it from your office.) Also, keep your phone charger handy, just in case the interview is going so well that your charge starts to run down.
Keep the end game in mind: Before phone and in-person interviews, I like to use this trick: Think of three characteristics you would like to portray during the interview (creative, analytical, polished, intelligent, interested, whatever!). Write those three attributes at the top of your notes, and as you glance down at them during the interview, you'll have a great reminder of how you want to present yourself.
During the Interview
Don't answer questions right away: Since you can't see the body language of the interviewer, wait a half-second before starting to answer any question. Sometimes, people ask questions and then keep talking, and if both you and the interviewer begin speaking at the same time, it can be awkward to figure out who should keep going.
Slow down: Speak slowly at all times, even a tad slower than you speak in real life. Phones can intensify the pace of your words.
Use your hands: It's okay to gesture while talking, even if no one can see you. Gesturing will make the call feel more like a regular conversation, which will normalize the situation and help to calm your nerves.
Say thanks: Follow up with a thank you, just like you would in a face-to-face interview. And I also think it's nice, if an assistant set up the call, to send a quick note thanking him or her for helping facilitate the interview