I agree with Mike.
We've been leafleting for 20+ years. We return to the same area every 12 - 13 weeks. Use a combination of Royal Mail, Leaflet distributors and shared with local community type magazines. All have a mix and a place. We're selective where they go and if we've not worked in an area from the leaflet drop for six months, ie two drops, we chop that area off the distribution schedule. We've used this to cut the numbers distributed without affecting the response, so save massively. It does take time to build, and you need an offer on with an expiry date to get the best response. The response is FAR better with an expiry date on the offer, not a 'for the next two weeks' but 'Only with this leaflet before the 31st August 2012'.
It works two ways, brings in new business AND also prompts existing clients and referred clients to call. If you ask every existing client where they got the number from, or what prompted them to call this time, the Leaflet will keep coming up. If you're new you obviously wont have those calls so it won't be as effective as if you're established.
Hope that helps,
Al.