Familiarize yourself with the format and directions of the TOEFL, whether it's the paper or the Internet-based version.
Listen to the English language with the help of audio compact discs, digital audio files or software. Many online retailers sell a wide variety of computer programs that teach English.
Read a reliable TOEFL study guide. There are many good guides on the market. The Official ETS Study Guide is your best bet, since ETS writes the TOEFL itself.
Practice taking the TOEFL under timed conditions, just like on test day. The more you simulate such conditions, the less likely you will be nervous when the time comes for you to take the actual TOEFL.
Hire a tutor, if financially feasible. A private instructor can correct any mistakes you are making before they become habits. A tutor will also serve as a motivator and confidant when you feel down about your TOEFL progress.