I often get asked this question, and my usual response is "Write the misspelled word several times until you develop some muscle memory and a visual image of the correctly spelled word." However, for the internet-based TOEFL, it doesn't make too much sense to write the word out longhand, since the iBT is an online exam. What should you do?
Now there is a wonderful online site called Spelling City. I tried it out myself and have also gotten feedback from one of my iBT students that it was fun and effective.
There are vocabulary lists of words arranged in alphabetical order for first through eighth grade, and you can either study the words or test yourself. In either case, you can hear the individual word pronounced or listen to the word within the context of a sentence. In addition, you can create a list of your own commonly misspelled words; if they're in the 'bank' of words at this site, 'Spelling City' will help you to practice those particular words.
I'm not sure how long this site has been in existence, but I'm very happy it's here and that I have a suggestion for how students can improve their spelling in a fun way.