Open up your word processing program of choice and create a new document. At the top of the document, in justified right type, add your name, address, email, phone number, and the date. Drop down one line and with type justified left, add the recipient's name, company position and address. In your salutation, address the person you're writing to by their last name, unless you're on first-name terms with them.
Use the first sentence to introduce yourself and to state the intent of your letter. If you're following up on a previous meeting or phone conversation, mention it in this opening paragraph. Be sure to also touch upon the genre of your music, and mention any promotional materials you're enclosing with the letter, such as a CD or photographs.
Mention any positive reviews that your music has received, and include one or two pertinent testimonial quotes to back it up. If you're a touring artist, this is also a good time to invite the recipient to any of your upcoming live appearances. If you're purely a recording artist, then include links to your website or blog. This will show that you're an artist who takes your business seriously.
Write your closing paragraph, making sure that you politely thank them for their time. Be upbeat about hearing back from them, and mention that you'll also be following up with them via email or phone.
Sign off by typing "Sincerely" and then leave several line spaces before typing your name. In this space add your handwritten signature. Make sure that your contact information is clearly visible on all of your promotional materials (in case they get separated) and add them to an envelope with your letter. Before mailing, make a note of the date of your submissions and responses, so that you have a record of all outstanding correspondence.