As soon as Emberly seemed to have the comprehension skills, I talked to her about how her private parts weren't for anyone to look at or touch, nor should she be made to look at or touch those of anyone else.
She's always seemed to understand the concept, but with horrible stories in the media and anecdotes in parenting communities, my anxiety level about her ability to apply the concept outside of our home rose. Now that she's a bit older, I wanted to drive the point home without scaring her or making her anxious. She loves to read, so books seemed like the obvious choice.
We read the following and enjoyed them all:
I said No! A Kid-to-Kid Guide to Keeping Private Parts Private, by Kimberly King. This book is told from a kid, who talks about the roles different people play in our lives and "red flag" situations and people. It was the most interactive book, with prompts for discussion, but it's also a little long, so while it is good for the patient preschooler who enjoys reading and discussion, for most, I think it's probably better for the 5-8 range. Emberly loves to imagine scenarios and what she'd say and so forth, so while she did get little antsy, we finished the book in one sitting.
Those are MY Private Parts, by Diane Hansen. This one had a rhyming scheme that made it fun to read and catchy. The illustrations, while cute, were kind of distracting. Emberly couldn't resist interrupting to ask questions about what parts of the drawings were supposed to be. Less of a issue the more you read the book obviously.
Some Parts are NOT for Sharing, by Julie K. Federico. This was really short and basic. I would have been disappointed if it was the only one we read, but it complimented the others well. I think this would be good for even younger children as well.