Get a professional photo taken of you or your band. If a picture's worth 1,000 words, then a good photograph of your band will speak volumes. Try to let your band's personality come through the picture, while still maintaining a level of professionalism.
Write a cover letter. This will be the first thing that's read by your potential employer. Make sure to address each cover letter to the specific person to whom you're sending the portfolio. Never address a cover letter "to whom it may concern." Include a short blip about yourself or your band, including your style (compare yourself to other similar artists) and what you want from your addressee (whether it's a gig or a record deal).
Include a biography of yourself or the band. Talk about each member specifically, and the band as a whole. Speak about who plays which instruments, what songs you cover, how the band came to be and any awards or achievements (such as radio play or tours). If neither you, nor anyone in your band, is a skilled writer, then it may be best to hire an expert to write your bio. It's important to keep your portfolio as professional as possible.
Include a professional compact disc recording. It's very important that the recordings of your band accurately reflect the image that you're attempting to project. If you submit low quality recordings, it will show that you're still an amateur, and your new potential employer will be reluctant to hire you. While it may cost extra money to buy good recording equipment or rent studio time, it may be well worth it in the end.
Include contact information. Whether this is a business card or a small insert that's located inside the CD case, it's necessary to leave a way for your potential employer to get a hold of you. Include the band's name, as well as a website, email address and phone number for each member.