It starts way before you are running over budget. Your entire sales and contracting process needs to reinforce the idea that there are no fixed cost estimates prior to the completion of the discovery process and sign off by the client on the wireframes and/or software specification document.
Granted, 90% of clients don't really work that way. In which case you need to gauge your comfort level with getting them to buy in, or in some cases, you simply have to walk away from the client that has a one page overview of a complex web site and wants a fixed cost, binding bid.
Granted, 90% of clients don't really work that way. In which case you need to gauge your comfort level with getting them to buy in, or in some cases, you simply have to walk away from the client that has a one page overview of a complex web site and wants a fixed cost, binding bid.