(Experienced TOEFL tutors - please add your top reasons that I haven't mentioned here...)

Many people stare at their low TOEFL scores in disbelief, "What happened? I was expecting more than this...!"
Avoiding the following most common mistakes in TOEFL preparation will help you escape such a fate:

1. Reviewing English in general, without any attention to the specific setup and structure of the TOEFL test itself
Brushing up on your English skills is great, but you need to know the specific question types that you will see on the TOEFL test.

For example - Did you know that you will be asked to speak about your opinion for or against something in less than 45 seconds, with only 15 seconds to prepare? Do you know how to maximize that 15 seconds to prepare a quick outline for an organized and cohesive response that opens a topic, develops it, and brings it to a conclusion in just 45 seconds? This is not something you would be prepared for just because your level of English is generally good.

Another example - Do you know the trick to answering the "black box" question on the Reading Section? If you are saying, "What is the black box question?" then you really need to pick up the ETS Official Guide to the TOEFL Test and go through it with a qualified TOEFL tutor!

2. Reviewing TOEFL test strategies exclusively, without enough language skills to perform well in general, or without enough general English review
TOEFL is an English test, and you must be proficient in the four major uses of English (Reading, Listening, Speaking, and Writing) to do well on the TOEFL in general. You will need a good vocabulary and understanding of grammar, as well as some overall fluency in both written and spoken English.

While knowing the best TOEFL test strategies and inside tips may improve your score by 10-20 points, you won't be able to jump from a 50 to a 90+ without solid language skills.

3. Using preparation materials that do not give you real TOEFL questions and time limits
Some TOEFL preparation materials online offer "TOEFL quizzes" and other activities that have no focus on any specific TOEFL skill. They are just English activities that are not related to the specific skills needed for the TOEFL.
For example, a "TOEFL Vocabulary quiz" that asks for synonyms (similar words) or antonyms (words with opposite meanings). This is simply not a TOEFL question!

Another example would be a so-called TOEFL grammar tool that asks you to identify the parts of speech in a passage, by underlining the subject and circling the predicate, etc. TOEFL does not test your knowledge of grammar terms at all.

Important note:
Some great TOEFL preparation books, like Cambridge or Bruce Rogers, include these kinds of activities, yet they are done in a series of skill-building lessons that lead you into real TOEFL situations gradually.

The bottom line:
If you are not preparing for the TOEFL Reading Section by studying 700-word academic passages, answering 12-14 questions on them, including the same variety of questions that will come on the real TOEFL, in a 60 minute time limit for three passages together (or 80 minutes for four passages), then you are just not preparing for the TOEFL Reading Section properly.

If you are not preparing for the TOEFL Listening Section by listening to short five minute clips of academic material and conversations about college life, and then answering five or six questions about each passage, including the same variety of questions that will come on the real TOEFL, then you are just not preparing for the TOEFL Listening Section properly.

4. Relying on a guarantee offered by a TOEFL tutor or preparation course
OK - great! You've found a tutor or a program that guarantees that you will get at least an 85, or they will refund your money. So if you get a 70, you can get your money back! This may actually de-motivate you, putting the workload on the program itself or the instructor to improve your score.

Realize that money-back guarantees are marketing gimmicks. No one can promise you certain results. A good, realistic teacher or language center will help you set a realistic goal for yourself based on diagnostic tests, but they will not promise you certain results.

You need a solid TOEFL preparation program, an experienced teacher who knows the ins and outs of the TOEFL, and lots of personal hard work to get the score you need. And this is one time when a refund is really no consolation! You get your money back, and... OOPS - You're not going to college!

Some people may rely on the reputation of the school or teacher that helps them prepare for the TOEFL. This is a similar trap. The best TOEFL program taught by the greatest experts will not help you, unless you work hard within that program. A certificate from a top language center that declares you absolutely 100% prepared for a 90 on the TOEFL iBT means nothing until you have that 90 on the ETS report!

5. Preparing for the TOEFL in one's native language
Do not buy TOEFL preparation books written in your native language! Do not attend a class that teaches TOEFL strategies in your first language!

It's really simple - At this level, you need to learn English in English. And you need to prepare for TOEFL in English. Language experts everywhere stress the necessity of learning a second language in that language itself, not basing it on the first language.

You lose precious opportunities to improve your English fluency by studying in your native language. Sure, it seems easier! Sure, you may be able to understand some strategies more quickly, but - quite frankly - you are refusing to take your English to the next level this way.

6. Not practicing with the appropriate skills to prepare for the four TOEFL sections
Some TOEFL preparation materials ask you to write out your entire spoken responses before you speak!
HELLO! Its the Speaking Section, not the Writing Section! The sooner you start recording your 45 second and 60 second spoken responses to actual TOEFL questions, using ONLY the 15, 20, or 30 seconds given for each task, the better.

...You should not try to write out a full and complete answer. There will not be enough for you to do that, and raters want to know how well you can speak in response to a question, NOT how well you can read aloud from something you have written...

- ETS Official Guide to the TOEFL Test (p.167)

Another common mistake is reading scripts of listening passages to prepare for the TOEFL Listening Section! Once you read the script, because you were too lazy to get the CD and play it, or too cheap to buy the audio CDs that came with your course, you change the activity from listening to reading. So make sure you listen to listening passages when preparing for the listening section, and stay away from those transcripts.

Sadly, the ETS Official Guide to the TOEFL Test (pp.127-139) actually makes the mistake of introducing listening tasks by asking students to read transcripts of lectures and conversations, without providing the actual audio files!

Its really a no-brainer
- Practice for the TOEFL Listening Section by LISTENING (not reading), and practice for the Speaking Section by SPEAKING (not writing)!

... To be continued... (maybe)