A Most local radio stations will be able to help you put together a vaguely professional-sounding ad, normally by outsourcing the job to a favourite production company. If you listen to some of the ads the station has produced for clients in the past, you should get an idea of the competency of the agency or agencies the station uses.
Think hard about which slots will suit you best. The breakfast show will have a different listener profile to the lunchtime programme or a late-night show promoting new music. Expect the station to have carried out detailed research to explain its audience.
Aside from production costs, radio advertising is usually priced in terms of "cost per 1,000 listeners". For example, a 20-second slot might be billed at £5 per 1,000 listeners, so a slot on a show attracting 20,000 listeners would cost £100.
Experts say the golden rule of radio advertising is to mention the business name three times and the offer twice. For inspiration, check out the Radio Advertising Bureau's archive at rab.co.uk, where you can listen to some of the most effective and amusing recent ads .