How to convince a user to fill out a survey

I am working on a project that provides a service (landscaping) to members of the community. When the work for that client is finished we always encourage the client to go to our website to fill out a survey to review the work that we have done.

The problem that we have been having is simply that we are only getting around 20% of the customers to fill out a survey, and only about 10% of those have anything useful on them.

In the past we have tried to convince customers to give use feedback on these surveys by offering a sort of e-coupon sent to their email. This only generated about 20% more surveys with only 5% being useful and the rest being something like "Good service guys".

My question is, how do I convince customers to fill out our surveys, and fill them out with useful comments? Do we need to offer a coupon again, but only for "useful" surveys (how do you judge something like that)? Or should we offer a certain % off their final payment? Or are there others ways of convincing a customer to fill one out?