Test-takers ask the Pro this question all the time.
"Dear Pro. How can I get a high TOEFL iBT score?"
Answer #1: Read, read, read. Read English novels, magazines, newspapers, dictionaries, essays, poetry, lyrics, graffiti, advertising, recipes, computer instructions, soup can labels, this blog, everything.
Answer #2: Learn to how to write a proficient independent essay. An independent essay is an opinion-based essay. Don't write a couple of independent essays. Write many. Write hundreds. Thousands. Millions. Write until you are a TOEFL Pro.
Remember: The independent essay is the center of the TOEFL universe. I can't stress this point enough. Understanding how to write a proficient independent essay will unlock the mystery that is the TOEFL iBT.