Maintaining an open dialog and showing flexible and adaptable responses will be the best way to maintain the relationship. Also provide regular status reporting on items that matter most to the client. Finally, it is always best to define up front the "rules of engagement". I always set the expectation that changes will be managed (and proceed to explain HOW they will be managed).
Remember, change requests can represent financial opportunity, so I wouldn't necessarily shut them down or consider them evil. When I was a PM for imaging projects, at the implementation meeting after the signing of the contract, I would explain the major options to the client and listen to their requirements. I would often come back with defined changes that were up to 25-50% of the contract value, based on new requirements which were not known at the time the contract was signed.
Each change was represented by a change request which documented the change, the cost and schedule for the change, and other pertinent information. I would review these with clients and they liked the opportunity to be involved. Sometimes they would authorize all the changes or they would pick and choose -- some changes were made immediately and then some were deferred to a subsequent implementation phase. This provided an opening to continue the project when the main work had ended.
As I reported status, I tracked their initial contract requirements separately from the changes. A mini Earned Value Report showed them where we stood each week. Once they understood the report, I never had an issue collecting on an invoice.