By writing instructions to insist that it double verifies every (non obvious) claim with a minimum of two independent sources. I also told mine to always assume that the initial prompt is missing crucial context, and to ask as many follow-up questions as necessary until it has enough information to provide the answer to the question I'm really asking. (For speed and efficiency you can even make it give you multiple choice options to click on.) Because sometimes the problem isn't with the LLM, but with the user asking the wrong questions.

Using those two instructions alone, I've encountered considerably fewer hallucinations, and when I'm still not certain, I can simply click on the sources linked next to every single claim the AI makes.