You'll find differing opinions on the usefulness of highlighting. It's true that if you indiscriminately highlight entire passages (maybe even the whole book), the highlighting won't help much when you go back to review the main concepts. Also some anti-highlighters say that highlighting makes for passive rather than active reading. This is similar to jotting down everything the instructor says but without making sense of it yourself.
It is possible to use highlighting effectively, but when highlighting, it makes sense to do so thoughtfully. Consider these guidelines for highlighting:
Focus on the main point -- and that may not be the entire sentence. It's perfectly okay to highlight only key terms or parts of sentences. In fact, you may get a better sense of the main idea of a paragraph if you highlight a string of words (excluding extraneous information) that lets you glean the main idea at a glance.
Consider reading the entire paragraph, and then going back and highlighting the important words and ideas. If you highlight from the start, you may not be sure of the paragraph's purpose and how to best capture that purpose or idea with your highlighter.
Don't make highlighting more complex than it needs to be. Some students use several colors of highlighters to call attention to different types of information. This is overkill and is likely to add confusion (rather than clarity) when you review this information. Also, this makes taking notes more time-consuming.
If you buy a used textbook or other reading material, look for one with little or no hightlighting. It's hard to ignore the previous owner's highlighting.
In addition to highlighting, consider jotting notes in the margins, next to passages.