Many moons ago when I was an undergraduate we had a tutorial on metabolic pathways in the body. To make it relevant and to stop us falling asleep our tutor decided to look at how alcohol affects your metabolism.
To cut a long story short alcohol blocks one of your metabolic pathways so you get a build up of toxins in your body and you feel terrible.
The best replacement for the missing substance is fruit sugar. This can come in any form. A glass of orange juice for example will help to replace the sugar or even jam on your toast the next again morning. Another way is to buy fruit sugar (available from most supermarkets) and sprinkle some on your cornflakes. This helps your body to start working properly again and for your metabolic pathways to start breaking down the alcohol. Remember alcohol is a toxin and your poor body is trying desparately to get rid of it.
Obviously drinking water before you go to bed helps tremendously as it rehydrates your body and helps to flush the toxins out. Its does help but its not a quick fix - then again with a monster hangover what is?!