Production Matters. There is a big difference in "demo" and "master" quality. Music supervisors only choose "master quality" recordings, and these can be home recorded, but must have a quality pro mix. They frequently ask for songs with fresh, unique instrumentation and arrangements beyond the traditional basic guitar, bass, and drums. Additionally, they rarely choose songs where the vocalist has a strong accent, unless they're looking for something very specific (like a swamp song for True Blood). If you're no instrumental savant, that's okay! Find a great producer who can give your song the most current sound and be sure to specify that you want master quality recordings and a singer with no identifiable accent. That way, your song has the widest range possible for use in different markets.
Be Positive. Music supers, especially those for major networks and those in advertising, constantly need positive, uptempo songs with simple, happy, optimistic lyrics. The next time you're watching TV, pay attention to the commercials for companies like Target, JC Penney, Coca Cola, or AT&T. Listen to the music – it's almost invariably simple, happy, uptempo songs. That being said, there is a current trend hitting of "dark, moody" cover songs that many Indie artists are cashing in on. But don't submit a cover song unless they're asking for cover songs.
Let Your Emotions Rule. When writing lyrics for film/TV, aim for lyrics with general emotional content as opposed to detailed imagery and evolving storyline. For example: if the scene is a montage of a couple walking on the beach, snuggling in front of a fire, kissing, then supers will choose a song with lyrics like "I'm so in love, feel so wonderful" before choosing "We're gonna get married and have 3.5 kids and live in a log cabin in the mountains" because the second lyric describes an action that isn't occurring onscreen.
Metadata is Your Best Friend. One of the most important things in pitching to Film/TV is making sure you encode the MP3 you pitch with your contact info in the metadata. Sounds complicated, but it's easy. I'd recommend using iTunes, mostly because the dozen supers I know all use iTunes as their library. Using a different program may show up differently in their iTunes and not show all the info needed. Your lyrics also can go on the MP3 using iTunes. Very important point don't overlook doing metadata right!
Be Prepared, Be Low Maintenance. Music supervisors are often on a short time schedule and make quick decisions and will always choose a song that is pre-cleared for Film/TV. If you plan to pitch a song for placement in commercials, movies, or TV shows, make sure you get a Film/TV license from the producer. With most studios, it usually adds a little extra to your production cost, but it's well worth it to be able to tell an interested super that it's ready for placement and can be used right away without the time delays of gathering additional paperwork. That is almost ALWAYS a DEAL-BREAKER and may also be the end of your relationship with that super.