This set of posts comes from Pastor Mark Driscoll and his wife Grace. The sermon series The Peasant Princess brought up dozens of questions that they felt they should respond to. The result is this blog series: Christian Sex: Frank Answers to Honest Questions.
Answer: First of all, encourage them by noting the physical and non-physical reasons you are attracted to them. Then, you can ask them if there are ways you could be more attractive to them. This will help to place any suggestions for improvement in a more positive context. You can then proceed to lovingly explain ways in which they can become more attractive to you. Examples may include a new hair cut, dietary changes, exercise, particular grooming, wardrobe updates, etc. The key is to continue encouraging any changes they make and be open to making some yourself.