to make it as accurate as possible try to do the same thing each week
myself I would have drinks shakes as normal and then do the same each week - that way week on week you will get an accurate show of your loss
Please don't worry at all about having your shakes or water - weigh in's shouldn't disrupt the diet. What you weigh at that particular time doesn't really matter as long as you stick to the diet, because you'll lose weight anyway. A very wise person on here told me that ages ago and my new CDC said the same thing.