I give my clients their full edited images on CD. How can I ensure that they will print with me and not go elsewhere?
I know the question has been downvoted, but I think it deserves an answer.
how sure am I they will print with me
Not at all. By giving high resolution images without watermarks, you are forfeiting any control method that you might otherwise have been able to use. People will therefore go with what they see as cheapest/best value.
Some people will go for cheap, some will go for better quality. The issue with this is that it's their interpretation of quality/value for money, and not yours.