Since recently OpenAI have introduced custom instructions where you can provide specific details and guidelines for chats (see https://openai.com/blog/custom-instructions-for-chatgpt )

What have you used?

At first I added a lot, but I toned it down because I felt chatGPT would sometimes focus on that unnecessarily.

Now I only have my country and languages, and I have requested any answers to be in metric units. That last one is very useful when I ask for recipes, before it would talk about Fahrenheits and Ounces and what not. Now I at least get Celcius, milliliters and grams!

I also added that it should reply as an expert in the field with a casual tone. Which causes it to go "aah, the old ..... problem" whenever I ask it for debugging help :D