Shazzie recommends a litre of warm water with the juice of one lemon. This worked better for me than a mug with a slice of lemon in it. I'm sipping this first thing in the morning and then waiting about 20 min+ before I eat anything, as of course too many fluids are supposed to impair the digestion of food. This is supposed to be good for your liver.
There are lots of health benefits, namely peristalsis! Improved skin and a diminished appetite are other bonuses. It tastes lovely with ginger too. I use ginger if I'm out of lemon.
Your body is super thirsty first thing, so it's easy to overeat if you don't first drink water.