"Artists Beware"

It's always a good idea to write a revisions policy, something that gives examples of what you consider to be large revisions that would require additional payment, and maybe set a number of small revisions you are willing to do without charge. Encourage commissioners to read this policy before they commission you, and if what they're asking for starts to come into conflict with it, refer them to it again and warn that you will start to charge for revisions. If they're still insistent on clashing with your policy, it's easier to present them with it again and call the commission off, keeping whatever you're owed for the work you've done.