Show me the Money.
At my own dear school, I have the unofficial role as the"Scary Grad School Guy." I give a number of talks each year on how hard it is to get into a good grad school, how hard it is to survive grad school, and how hard it is then to get a real job.
Why is this necessary? At the root is that most of my students, even the best ones, only have the vaguest idea what is going to happen in grad school. They don't know what a graduate syllabus looks like. They don't know how a real seminar is run. And they haven't the foggiest notion of what is expected of you for your comprehensive exams. Despite this, fully half of my undergraduate history majors say they want to go on to grad school. By the end of my talks, I can usually cut this number down to about 5%. Just passing around a few sample graduate syllabi probably takes care of half the job. Tim Burke's outstanding essay"Should I Go to Grad School?" also helps.
A critical component of my whole spiel is this: Don't go to grad school unless they pay you to do it. And by that I mean a full ride: tuition, fees, and stipend. Students should look upon grad school as a job... and you don't want to pay your employer for the honor of working long hours. If you are just going for an MA which is demanded for your existing job, that's fine... cough up the dough. But, really, nobody should self-fund grad school as a means of getting into academia.