It's been a few years, but I submitted several teaching pieces to various piano music publishers several years ago, and ended up selling a few. What I did basically, was prepare sample packets of my work with a cover letter. I also provided a performance tape with each packet. I had previously written to all the major publishers asking about submission guidelines and to whom unsolicited materials should be sent to. Then, I just started sending the packets on a 6 week rotating basis. I would keep track of which packet I sent to whom and when.
Keep in mind, that often, when a person's work is rejected, it's not because it wasn't good, but that it didn't meet that publisher's needs at that time for their market. As the market changes, so do these needs. What may not appeal to one publisher may appeal to another. You may notice how some publishers seem to gear their offerings toward a more specific styles of music while others are more varied in their materials. Some like more traditional things while others lean toward more unconventional sounds. It's important to get a feel of each publisher's bent. Often, companies will even tell you what they're looking for for future publications.
Although I'm always composing music for my students, I haven't taken the time to vigorously push my music. It takes a lot of time & organization. The squeaky wheel gets the grease as they say. Companies prefer to go with the composers already known. Afterall, they want their offerings to sell. They take a bigger risk with no names. But, if you keep plugging, someone will bite.