My best advice is sprinkle a bit of salt into half of the water you drink. Going low on sodium is a fast way to get yourself killed and cause some serious damage. Basically keep the salt intake up--you can deal with a bit more than required but you can't deal with none for a week. Other than that, I'd say eat a lot of fruits and veggies. Don't stress yourself out and don't push yourself. Be prepared for hunger cramps after the first 2-3 days. Once you start eating again, go with the BRAT diet for at least a day (bread, rice, apple sauce, toast) before you start on meats.
My personal advice would be don't do this unless you have a qualified medical professional (read: doctor or nurse) who can stop this at any time. If you hit a stressful spot or have something big (such as a large test) going on, don't try to fast through that.