On days 1 and 2, 4ltr of water seemed almost undoable - on day 6 today, and I've had almost 4 already.
I keep a 1.5 litre bottle (one of my old Oasis ones
) with me all the time, and aim to drink 3 of them in a day = 4.5ltr.
I keep a full bottle next to my bed as well, and make sure that I havea good gulp before I go to bed (I havent woken up needing to use the loo during the night so far) and try and drink about 1/2ltr before I get out of bed and dressed in the morning.
It really does just become second nature - every 5 mins, just take a swig
PS Yes, headaches are 1 symptom of Carb withdrawl - the more carbs you had in the few days before you began your VLCD, the harder the first few days will be.
PPS a headache is commonly caused by dehydration, but the actual brain itself cannot feel pain - it has no nerve receptors. A headache is actually caused by the aching muslces around your head and neck - it just feels like its your brain due to their placement.
^useless info gained from watching too much QI