Voice over work is highly sought after since it doesn't rely on looks and because it is purely fun. The hours are much shorter than typical set work days, there's no make up and hair, and the crew is considerably smaller. However, getting the work is the hard part.
Take voice over training classes. While a lot of people may think these are silly they are essential to increasing your bookings. These classes teach breathing techniques, microphone logistics, dialects/accents, audition preparation as well as acting.
Make a demo tape/CD. Include television and radio spots, animated characters, narration and/or industrial work samples. You can write these yourself, find old audition scripts from other actors or use material currently on the air but I strongly advise against this.
Talk to your agent when you have an audition. Know what to expect in the audition. Get as much information about your character and any available sides prior to your audition. If the sides are available see if they can be sent to you. Read the script several times. Get to know what you are saying and understand your character. Have a friend rehearse with you and give you a bit of coaching.
Show up to the audition at least fifteen minutes early. Take a water bottle with you and keep your throat hydrated. Go in when called and be polite to the casting director and/or assistant. Adjust the microphone as needed for your height. Listen to the casting director's directions.
Slate your name clearly. Pause for a moment then start into the script copy. Remember to enunciate and stay in character. If the casting director gives you corrections take those and redo the audition. Ask questions if you don't understand what is expected of you or the proposed changes. Say thank you and promptly leave after you're done. That's it. Hopefully you'll get a callback and land the job!
Learn as many accents as you can do well because it expands your possible castings.