Answer:
Negative tags expect a yes answer.
I have met you before, haven't I? (expects a yes answer)
Positive tags expect a no answer.
I haven't met you before, have I? (expects a no answer)
More examples:
The application was refused, wasn't it? Yes, I'm sorry it was.
The guests won't be arriving today, will they? No, they are arriving tomorrow.
It's a beautiful garden, isn't it? Yes, we have worked hard.
You are not working very hard, are you? No, I'm not feeling well today.