I think we do specific measurements on certain areas eg. bust, waist hips. It doesn't matter where you measure really as long as you do it in the same spot each time. So if you measure your bust always do it around the nipples, if you measure your waist then do it around your belly button so that you know you are doing the same spot each time.

Also with weighing yourself, it doesn't matter how/when as long as you are consistent. So if you do it every Monday morning when you wake up, with no clothes on and before breakfast then do the same thing each Monday.