Highlight the answer, not the paragraph
A highlight is useful when you know why you saved it. Select the smallest passage that contains a claim, definition or example. If you highlight four consecutive pages, you will probably need to reread them later anyway.
Add one sentence in your own words
Write what the passage means or why it matters to your question. For instance, under a discussion of sleep, note author says recall improves after review, check cited study. Avoid treating the author's words as your own summary.
Keep a source reference
Record the book title and chapter; add a stable page reference for PDFs. Reflowable EPUB page numbers can change when you adjust text size, so the chapter and a short quotation may be a better reference.
Review outside the book
Export or copy your notes to a document when supported. Group them by question, not the order you encountered them. Compare two authors discussing the same subject and write where they disagree. This is the step that turns a reading tool into useful study work.
Check your backup
Highlights often live in the reader's own database rather than inside the EPUB itself. Before switching phones or apps, test the export of a single book and make sure both its note and source reference survive.
You can put selective marking into practice in Readream Basic: highlight a short passage, then revisit it after the chapter to decide whether it still matters.