Enroll in a jingle course. There are courses for practically everything else, why not jingles? Jingle E-Course +PLUS will not only teach you the elements of writing a successful jingle, but also so how to sell your commercial jingle.
Call the department of music at surrounding universities and talk to professors and music instructors. They may have ideas on how you can sell your jingle.
Contact places that specialize in voice-over or teaching voice-over. Colorado Free University (which is not free) in Denver offers voice-over classes. Jingles for commercials need voice-overs, so this might be a valuable source for those wishing to sell jingles.
Upload your jingle on youtube.com and promote it. Show it to the company you want to sell to so they can see your success, giving you a better chance of making a sale.