If I can add to your tip, here are some extra radio advertising tips that being in radio for 25 years taught me (in addition to several boxes of awards for radio copywriting that sit in my basement)...

1) Don't allow your client to do his own radio commercials. Unless he's such an amazing character that he has universal appeal.
2) Don't write "conversational" spots unless the conversation sounds completely natural. Friends don't give friends business addresses, phone numbers, directions, or positioning statements in a natural conversation, no matter how strenuously they're recommending a business or a product.
3) Don't use background music unless it's relevant and adds something to the commercial.
4) If you ever use the words "for all your [x] needs" in a radio commercial, please hit yourself in the head with something sharp. I've never met a good writer who uses that phrase, much like there's never been a good commercial that contains the phrase "but wait, there's more!"
5) Comedy is hard to write. Don't do it unless you really know how.
6) Don't put directions in a radio commercial, or an address, or a phone number or web address, unless they're REALLY easy to remember after hearing them ONCE while dealing with all the distractions that come from driving a car that's moving 70 miles per hour. Why assume that your prospective customer was listening, with pen in hand, eagerly waiting for your commercial to come on so they could write down your information?
7) Develop a hook, positioning statement or theme that you use in EVERY radio commercial you EVER do.
8) Remember that every spot you ever run simply adds to your prospect's ongoing mental file on your business. They'll remember the great spots just as much as they remember the ones that sucked.
9) There are plenty of guys like me out there who would be happy to write and produce your radio stuff on a freelance basis for FAR less that you'd expect.

That's a good start. I'll bug you with more later.

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