Here are three more principles that are perhaps secondary to the five listed belowbut of great importance also.

1) The intellectual history of one's discipline.

You should know something about the history of other people doing what you want to do. In sociology, you might want to read Durkheim and Weber. Some disciplines are oriented strongly toward the present, or the very immediate past. It might take a while to gain a historical perspective, since graduate school can be focused more on the "estado de la cuestión," the current state of research, than on the intellectual underpinnings. When you have been in the field longer, you have actually lived through more of the intellectual history of that field, and also, maybe, have been able to look back at things that happened before you joined the conversation.

2) Language.

Obviously I am biased since I teach a foreign language. I think it's valuable in any field to know at least one other language with some real depth, beyond the kind of very basic reading knowledge required to fulfill a requirement in Grad School.

3) General knowledge.

Knowledge of intellectual history, science and the history of science, art history and music, philosophy, mythology and comparative religion, linguistics, etc... It's interesting in my field how students coming from Latin America are often much better prepared in terms of general knowledge than students with their undergraduate education in the U.S.