First you want to make sure you give them something they will like. If they hate your thank you gift, chances are they won't offer what you were thanking them for again.
If you go over the top, then the person you are thanking will feel bad that they didn't do enough (I know I have!).
If you underwhelm them with your gift they might think you are cheap and there could be the same consequence as the first senario.
If you know the person you are thanking very well, then there isn't much of a problem, you just have to think and observe.
If you don't know the person well at all, then the gift takes a little more thinking and a little more effort.
I'm making a mountain out of a dish cloth, I know, but I care about these things.