Start with the question, not just the answer. A copied fact is easier to use later when you remember why it mattered.
Separate what you observed from what you inferred. If an idea came from a book or conversation, leave a reference beside it. A short pointer is often more helpful than a perfect filing system.
When you return to a note, add what changed. Keeping the earlier thought visible turns a notebook into a record of learning rather than a collection of conclusions.