Marking clearly where the instructions end and the content begins, using tags, headings or fences. It sounds trivial and it removes a lot of confusion, particularly when the content itself contains instructions or looks like a question.
Technical view
Explicit delimiters improve instruction following and reduce the chance that content is interpreted as directive, though they are a robustness measure rather than a security boundary, since determined injection defeats them. Models differ in which conventions they respond to best, with tag-style markup working well for some families. Structure also aids caching, since stable, clearly bounded sections can be positioned at the front of the prompt.
Related
Part of AI Terminology Index, section: Controlling and Extending Behaviour at Run Time.