As a web designer starting out, I encountered this issue on two different projects and pass along the experiences so other designers do not make the same mistake. The issue is this – when a client agreements to a web site design, be sure they sign off giving their approval in writing. Whether you devise a form or have someone such as a lawyer draw one up for you, the document should state two things. First, it should explain that by applying their signature, your client is approving the design as presented, with minimal revisions anticipated. Second, the document should state that any major revisions requested after the approval stage will cost your client additional money.
The first time this happened, I was designing an initial web site for a non-profit foundation board I was familiar with. There were problems with the project from the beginning, in that no one was offering design ideas or providing feedback on the three prototype sites I developed. Once they did, the board decided they liked a combination of two, explaining how they wanted the designs combined. With deadlines quickly approaching, I and several other board members understood this design was the one and I began building pages. However, as I started constructing the site, three board members met and decided they really did not like the design after all. Hours of work had just been wiped out. With approval in writing, everyone is more likely to understand the web development stage is underway and there's no turning back, at least not without additional cost to the client.
The second instance also involved a non-profit organization with a slightly different twist. Offering my time and expertise as a volunteer, I had provided an executive director with four designs, and he selected the one that best met his needs. Again, I neglected to have the client sign off on the chosen design. In this case I had not actually started building the site when I got a call that it just so happened the director had a niece with training in web design. She had seen the proposed site designs and wanted to give some feedback and revisions. It seemed odd someone was offering feedback after the client had selected their design, but with nothing in writing I had little choice but to go along. Again, had the client signed off as approving the design, he may very well have reconsidered having his niece review the designs or had her do so before choosing one.
Having the client sign off as approving the design applies whether you're working with a paying client or as a volunteer. As a volunteer, you may not have the option of charging the client extra, but you can make it clear any revisions requested after sign off will mean that you charge the client from that point forward. In either case the output is the same – extra time and work is required because the client changed their mind. With nothing documented you have no recourse but to make the revisions at the agreed fee or as a volunteer, to keep working on the project. As a volunteer it's easier to walk away from paid work, but you also have to assess if taking such action is best for your business in the long-term.
Once a client has given their approval for a web site design, insist someone in the organization with authority sign off, with the understanding that any major changes will result in additional costs. The person giving approval could have been a project or team leader, executive director, manager, etc. Give a copy of the form / statement to the client and keep one for your records. Not only is it good business, should the client decide they do not like the design you've implemented, you have a written statement of approval and a means of financial compensation for your work.