I certainly agree with you that it is a tough line to walk, especially since you spend so much time with them. Here are some of things I try to do or not to do. I try to keep the subject matter of conversations with students to only those related directly to class and the material. Additionaly I try not to minimize the amount of personal information I share with students. When I do share stories of a personal nature they relate to the material we are covering and usually about past situations I found myself in where the knowledge/insight I was providing them with helped me or could have helped me. I sometimes have to bluntly but nicely remind students that the student to teacher relationship has bounderies that must be kept. Most students completely understand and usually "step back" a little. I hope some of that helps. :)