I think it's all about the unique selling points you have in the business, and the angles you take, and a lot to do with keeping your customers your own. As if they've done you well for the last 5 years, they're probably all about to have another job done now so it just goes around again.
The likes of rate-ya-sausage.com type websites all geared at pointing out the cheapest trader have a lot to do with customers expectations.
I bet you lads know more than I do that some of these customers have got the cheaper one in, but could have quite happily paid the going rate had they not had that cheapo one quote, and the 7 quotes near the cheapest price.