It's always been my opinion that the best Acceptable Use Policy (AUP) implements both a quality technical solution and an appropriate educational/training program.
For the technical side of the approach I make it a point to ensure my clients understand that while it is true the issue of objectionable surfing cannot be stopped 100%, the real goal is to make it practically impossible for the casual offender and to deter/hinder the more serious offenders from even trying. Of course reviewing the appropriate Internet related log files and looking for trends will often provide the necessary insight that can lead to closing the door to anything questionable that remains.