In my opinion, self-study is the best way to prepare TOEFL because you have to search for resourses and find another benefical way to prepare that exam. And by this way you made a big efforts to reach your aim, finally you become a master on particular area.
For the TOEFL, IELTS, or any other critical exam, I would definitely hire a teacher.
The only alternative would be a book as follows:
Official Guide to the TOEFL Test With CD-ROM, 4th Edition (Official Guide to the Toefl Ibt) Paperback – August 29, 2012 by Educational Testing Service (Author)
I think that for the reading, listening, and writing portions self study may be the best way to prepare. There are a lot of great (free) resources out there. For instance, I use the great presentations on TED to make exam preparation materials for the listening test: http://www.enupgrade.com/listen-up/
However, the speaking portion of the TOEFL requires familiarity with a variety of question types and accents in order to understand and respond to question accurately. Of course, your pronunciation and intonation are a large factor of the scoring for this section. For this reason, I do not see how someone could study alone and self assess their pronunciation. After all, the TOEFL does not measure how well you talk to yourself. You can find more information about this part of the test here: http://www.ets.org/s/toefl/pdf/toefl_student_test_prep_planner.pdf
It might be possible to find a regular language exchange partner, but he or she may not be familiar with this test. There are several teachers on italki who are familiar with the TOEFL and can help you prepare specifically for this speaking section. You can check out my teacher profile here: http://www.italki.com/teacher/493889
My advice is if you did not attend any related English test class, at least few weeks professional English class is required to acquaint the orientation of English test. Otherwise, you might be a pretty smart and intelligent marvel. I am not a phenomenon in the least, so I took English class in cram school and learned with 2 native English teachers in the first place.
Bruce's suggestion is appreciated, you may seek for an answerable and favorable teacher to your learning, that would be a very good start.
I think it depends on your personality.