I have worked for Civil Engineering firms with a staff of 3, to a staff of thousands (Kimley Horn, 2400+ nationally).  I have seen a variety of incentives over the years, but the idea I thought was the most effective was more of a relationship builder which added an incentive to keep using that firm as their consultant.  Our Practice Builders (Licensed Engineers that had branched into their own client list that had may be several key clients that have the potential to become million dollar clients) handled the work for their clients with the energy and care needed to not just get the job done and permits in hand, but to go above and beyond expectations, which impressed the client.  One of the techniques I saw used with clients who became close, was to present the client with a special remembrance at key points in the project or at the completion.  An example I remember was a client who loved the Boston Redsox...in 2013 the latest year the Sox won the World Series, one of our office Practice Builders had traveled to one of the games and he bought a ticket for the client in a seat beside himself at the game...during the game he took a few photos with the client, got his ticket stub signed by one of the famous players and bought a jersey.  Most would stop at bringing their client to a game he had dreamed of seeing....and would have greatly impressed the client.  This practice builder went a step further and had a professional company frame the items he had collected at the game including a great photo of himself and the client in the stands.  He also had a silver emblem on the display commemorating the people and dates involved.  This served to cement the relationship with the client, which turned into a friendship between consultant and client...to this day, that relationship has grown to be a real teaming effort, which benefits each party.  Adding personal touches to commemorate events was by far the best relationship builder I've seen in 33 years in the business!