I just eat and drink normally on weigh in day. I weigh in at 6pm and usually have brekkie, lunch and sometimes a snack before I go. If you do the same thing every week it makes no difference at all, its when you vary things that it may make a small difference.
I know some people at the class I go to dont eat much at all on weigh in days - not something I could do and not a habit I want to get into. I enjoy my food too much. Good luck on your weigh in