Tip #1: Use the filters to exclude your bibliography and direct quotes.
Tip #2: Check that quotations marks ( "..." ) are used around every quote, an in-text citation follows the quote, and the in-text citation is matched to a citation in the References area (bibliography). Failure to do this properly can result in Turnitin highlighting these as matched text.
Tip #3: Overusing direct quotations can cause a higher score. Evidence is often best explained in your own words with an acknowledgement to the original author. Reserve quotations for times when specific language or information is given that can't be expressed in any other way. Technical terms, famous quotations, or concise statements that would change meaning completely if altered in any way are good examples.
Tip #4: Paraphrase well. If your own words are too similar to the original text, this will increase the similarity score. Learn more about paraphrasing at the CORE Library's Guide to Direct Quotations v Summary v Paraphrasing.
Tip #5: Some items just can't be fixed, such as strings of commonly-used terms or words. These are known as "small matches" and often appear on a report with a 1% match. You can't fix these -- but don't worry. Your instructor will recognize these for what they are, and it's why you are allowed up to a 15% similarity match!
Thank you to Monash University for sharing information from their LibGuide on Turnitin.