Commission a photographer to take some professional photos of you. While the crux of your portfolio will be the tapes you provide of your voice, it still helps potential employers to know what you look like and a little background about you. Include a resume in your portfolio, focusing on voice work, and any other entertainment industry tidbits as well.
Record your voice narrating a book. There are tons of royalty free titles available at sites such as Project Gutenberg, according to Voices.com. This is a great resource for beginning voice actors who don't necessarily want to give away their craft for free. Narrate a book to show off the clarity and tone of your voice. Choose a lively book that allows you some range in terms of different characters or scenes.
Join an acting group that specializes in radio theater. This gives you a good chance to further hone your craft and create recordings that you can then include in your portfolio. Stretch your vocal limits by taking on different characters that require unique voice-overs, from children's voices to the voice of the opposite gender. Inclusion of this type of material in your portfolio shows a potential employer your range.
Try your hand at public service announcements. Television and radio stations run PSAs often and usually won't mind having a fresh voice to do it. Contact your local TV and radio affiliates and offer your voice for a few PSAs. If they like your voice, they might offer you work more often. If not, you at least have a few professional recordings to include in your portfolio.
Amass all of your recordings onto one medium -- be it CD or memory card. This ensures that you don't have to provide several different mediums in your portfolio through which a potential agent or employer would need to sift. This makes your portfolio look more crisp and professional and makes it easier to send to a variety of potential employers.