The moment you start working on a translation, make sure that you keep your client involved in the project.
Be transparent and inform your client about your progress.
Get back to them to clarify text passages you are not absolutely sure about. Ask them for special terminology databases or glossaries they usually use to keep the consistency in their published materials. And make sure to find out the correct target group.
By involving your client from the beginning on, they will feel as involved as you are, and they will be more likely to come back to you for another translation project.