I worked for a year at my college's NPR station, and I'm a volunteer at my local station. Winning strategies:

1) Flat out sell the underwriting slots. On KMST, a pledge of $120 will let you submit a message to be read five times on a given day. Makes a great birthday/anniversary present.

2) Bring in really charismatic listeners and community leaders to pitch. KCUR has some excellent community pitchers. (And it's fun to run into people, hear their voice, and be able to say "You pitch for KCUR.")

3) Don't be ashamed to hawk the station's celebrities. KCUR has a patriarch of Kansas City's media host a daily call-in show. The highest one hour pledge total (I believe it was over $10K) came from offering a signed copy of his book with a $120 pledge.