When you've finished your script, time yourself reading it aloud, making sure that you keep within your limits. You can spice up your language if you want to, but don't knock yourself out. Chances are good that someone at the station will rewrite it anyway. Most stations have a public service director, or other staff person whose job it is to get PSAs on the air. Don't go to the expense or bother of finding a recording studio and having your PSA professionally produced.
Most stations try to maintain a particular quality of sound.
They are more likely to take a script and produce a PSA to their own standards than they are to run a pre-made spot. Commercial radio stations work hard to hone their listenership to narrow demographics, so your PSA will have a better chance of running if you tailor it to the station's format. These stations would be likely to run PSAs regarding children or educational events. Send boycott PSAs to college or community stations that don't run advertising. Here are some of the PSAs that RAN has sent out in the past to radio stations across the country.