To make sure that measurements are as relevant as possible, it is best to weigh yourself:- at the same time of day each time- on an empty stomach- with an empty bladder.  

It is not advisable to drink diuretic drinks such as alcoholic drinks or coffee before weighing yourself.  It is also not advisable to weigh yourself after intense exercise. Your glycogen (and therefore hydration) levels will be greatly reduced which will distort the measurement.  It is also very important to not repeat measurements several times during the day, these values are not comparable.