Practical advice from a UCLA professor for undergraduates contemplating PhD programs. Refreshingly, the summary includes some constructive advice other than "grad school is torture, don't do it". A few tips:
Graduate school is training in research. It is for people who love research, scholarship, and teaching for their own sake and for the difference they can sometimes make in the world. It is not for people who simply want more undergraduate courses. It is not for people who are in a hurry to get a real job.
Your undergraduate education will not enable you to decide whether to go to graduate school. You will need to ask for advice. You should figure that the decision will take about a year to make, so ask for a lot of advice over a long period. Start toward the middle of your junior year, if not before. You should get advice from everyone you consider either knowledgeable or wise, but particularly from professors.
Graduate school, as I said, is training in research. When a graduate school looks at your application, their principal question is, "Is this person going to be good at research?" Indeed, that should be one of your own principal questions as well. How can you tell if you're going to be good at research? Getting good grades in your undergraduate classes is important, but it's not really the main thing. The main thing is this: if you want to go to graduate school, you should start getting involved in research as an undergraduate. This fact is usually kept secret, but it's true.
The article has details on how to get involved in research, how to build relationships with faculty, and how to apply. See this prior post for more grad school advice. And here is a recent AskMeFi post on grad school blogs. (Thanks to Ryan for the top photo).