1. Realize that you might not be prepared. In your undergraduate program you were probably one of the smarter kids, so your professors liked you. But that makes you no different from any one else in your program. That does not mean that you are well-prepared for graduate work. It is very unlikely that you have read enough. I had never had a class in the BOOM so the summer before Grad school I read all the novels of Gabriel García Márquez, Julio Cortázar, and Mario Vargas Llosa. Later I realized a lot of Latin Americanists don't even like these writers, but that's another story. The undergraduate degree in Spanish does not prepare you for Graduate Work. It just gives you a small taste of what it's all about. The same could be true of other fields.
2. Realize that the profession you've chosen is brutally competitive. You won't get anywhere by just being an average student in an average program. Choose a field in which the ratio of PhDs to tenure track openings is reasonable. Like Spanish. Even in this case, you have to be one of the better students from a better program to get one of the better jobs.
3. Take care of the basics. If you are doing a PhD with a language in its name (Spanish, French) then master that language. You will be teaching it your whole life. If the language is English, then make sure you have "got prose." Mere competence is the key, because, look around you, not everyone is going to be able to transform themselves from bright undergrad to competent scholar in six years.
4. Begin building your cv at the beginning of your PhD program not toward the end. (Added after reading comment by Clarissa).