Let’s start with what you can offer your client. You can approach it from satisfaction from the shoot itself:
You love your photographs, or we’ll redo your session at no charge.
Or you can off a money back guarantee:
You love your photographs, or we’ll give you your money back.
Both are strong and show your level of commitment to your work.
Both also show your level of professionalism, and that your goal is to have happy customers.
Then it’s a matter of how you insert your guarantee into your marketing materials, and how well you stand behind it. Be ready to remind your customer of your guarantee throughout the process as well – sometimes it can help defuse a situation.