Choose 3 to 4 songs that epitomize your sound. The demo should include one or two radio quality tracks and one or two tracks to show that you have enough variety in your material to record a full-length album.
Make the first song the best song. The music should take no longer than 30 seconds to reach the chorus. The decision makers who listen to your demo must also listen to hundreds of other demos, sometimes daily. They do not have time to listen to the whole track. If you do not suck them in within the first 30 seconds, they will skip to the next track.
Pay a professional to master your demo. Mastering refers to the process of giving depth, clarity, and volume to your recording. A trained listener can immediately tell if the artist went through the trouble of mastering. While this is not necessary, it is invaluable in communicating to the listener that you are dedicated to your music.
Do not include any cover songs in your demo, unless you are a cover band. Iron and Wine may have blown up from their cover of "Such Great Heights", but this is not the time to show how well you play someone else's music.
Record your demo to a CD. Tapes may have made a comeback in some hipster circles, but save listeners the hassle of having to rewind or fast-forward a track.
Package the CD in a standard jewel case. Some radio stations prefer mp3s, but the majority of the industry still uses CD players to listen to music. A jewel case with a standard spine will visibly broadcast your name in a stack of CDs. This is important because cardboard or thin spine cases often become squished or lost in the stack.