Having managed a few DM (Direct Mail - adspeak) campaigns for companies and there are a few pointers I can offer
1 - Expect to have between a 1-3% response rate, any higher and you've done exceedingly well, on 500 leaflets expect 5 to 15 responses. This is if the message is one that people want to hear, which brings me onto...
2 - Target your mail precisely, not only the right business but the right person in that business, someone who makes purchasing decisions or heavily influences them and do it at the right time of month/year. Purchasing decisions happen within a company at certain times, find out when they are and ready your message to hit them when they are most responsive to it. This will mark you out as someone who is sensitive to a client's needs.
3 - Give them a reason to respond and a deadline in which to do it, so:
10% off all orders purchased before XX/XX/XX when you mention this leaflet. Do not be vague, be direct.
4 - Track your responses, so after sending the leaflets make a note of how many people mention them when calling, also remember to ask people as they will sometimes forget to mention where they saw your number. The reason you do this is you can then see how effective your ad campaign is. Response rate is important as you can then see whether or not you need to tweak, change, alter your method or message. It's the equivalent of server stats!
5 - People are lazy, forgetful, busy, or all three. So a courtesy call to make sure they received you mail is a must, even drop them an email after (I have a killer way to get people's email addy and have them opt in! They actually want you to have it hehehe (sorry can't share!

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6 - Never send the same leaflet twice, it reduces the impact of both leaflets and marks you out as a lazy company who never have anything new to say.
7 - Make your message about the customer's needs not your company's services.
So: You need XXXX then call us, we have XXX years expertise in solving problems like yours.
8 - As most politicians know, have one message and repeat it at least three times. A common device is to ask the same question so you can answer it.
So:
Servicing costs rocketing?
Want to keep servicing costs down?
Car running costs spiraling?
Focus on benefits not features. People buy hammers not because they like to own them or that they are heavy or look good, they buy them because they need to put a nail into a bit of wood. Therefore, Our Hammer is heavy, which means that you don't work so hard putting your nails in. "Which means that" is the phrase which turns every feature into a customer benefit, focus on those benefits
9 - Put your response mechanism all over the leaflet do not make the respondent hunt for it. If you want them to phone, make sure the number is boldly printed in a prominent position, not just tucked away out of sight on the back of the leaflet.
10 - Do not use photos unless they are good photos, stick to simple graphics if you cannot find a good photo. A bad photo will detract from you company like nothing else, if you absolutely need to have a pic then get a professional in who can light and shoot the subject matter expertly.
Follow the above and you'll have the perfect DM campaign, only question is, is this the best spend of your ad budget? What do your competitors do? Maybe they do that because it works for them? Maybe they have tried other routes and they don't work? Don't be afraid to go back to the fundamentals in any campaign? Why are we doing this? What do we expect? Will it work? What proof do we have that it will work? If you cannot satisfy those questions then maybe you need to look at the strategy again.
Good luck, DM is one of the trickiest things to do well, but get it right and the phone should start to ring!