People buy from people, not from institutions. Advertisers often forget this, and are more concerned about their perfect "image" than about relating to their audience as fellow human beings.
Source: Radio sales consultant Jeffrey Hedquist
It's not a perfect world, and listeners recognize this, so a commercial that tries to portray the advertiser as perfect, doesn't ring true.
Let the audience in on your little faults, the chinks in the armor. For example, the car dealer who says, "We have the best deals, the biggest selection, the friendliest salespeople, but...our coffee's not so good," allows the listener to discover his embarrassing secret. Don't be afraid to joke about your hard-to-find location, the tacky sign you inherited from the former owner, the boss's idiosyncrasies.
A little self-effacing humor can go a long way. Give listeners something to smile about. If a listener can say "Yeah, that's me. I've done that," you've established a bond. Now your audience is involved.
Poke fun at yourself and punch up sales.