I've done water fasting for as long as 28 days (twice), 21 days (twice) and shorter periods of time (7-14 days multiple times).
Before doing the fast, I did a LOT of research and bought a bunch of books. Avoid the religious fasting books. Go for the ones that are more scientific. Dr. Joel Furhman's book Fasting and Eating for Health was absolutely great in this regard.
Getting prepared - I got prepared by lightening my food load a few days before the fast. Moved to more fruits and vegetables and drank copious amounts of water. Its a good idea to do this anyway, since your bowels will basically stop functioning during the fast!
Starting the fast - This was the most difficult part for me. In fact, I broke down and stopped many fasts during the first 3 days because I could not handle the feelings of hunger, headache and so on. A few times I took Vitamin water - BIG mistake. It's best to stick to plain distilled water. This worked best for me during the successful fasts.
Cruising - After the first 3-4 days the bad feelings generally passed and I can only describe a feeling of euphoria. Yes I was incredibly tired. I had to get up slowly to avoid being dizzy. But you regain your strength pretty quickly and to be honest, I'm a programmer by day -- these times when I fasted have been the most productive of my life because of the mental clarity that I experienced.
Ending the fast - You have to slowly (really slowly) end the fast since your stomach will shrink. Keep it to mostly fruits and vegetables in the few days it takes you to go back to eating. As for going back to eating, you want to just change your diet to more fruits and vegetables anyway. Eating pizza and other fast food is a recipe for disaster. After my first ever fast (21 days), I ate 2 peaches. 14 years later I still remember the taste of those peaches, where I bought them, who I was walking with and what the weather was like -- it was that vivid and tasty.
Cravings - About 10 days in, your habit system starts to kick in, and many times I caught myself preparing meals -- just due to habit! I was not hungry, just the simple mechanics of preparing food for dinner took over. You can destroy a fast easily with just this muscle memory.
Benefits - I got numerous benefits, chief among these was blood work that was literally off the charts in a good way (did intensive blood work after one of the fasts). My doctor said he had not seen blood work that good in a long time. Mental clarity as I said. Weight loss that was permanent. Dramatic lowering of blood pressure.
Weight loss notes - I'm a heavier person. I went from 311 points down to the mid 200's and have maintained that. However if you want to have sustained weight loss you have to fast for longer periods, approaching 30 days, since as many others in this thread has rightly said, you lose muscle memory in the beginning. It's only after ketosis really takes over for a lengthy period (After 14 days) that your body starts to consume fat instead of muscle sparing.
I'd recommend everyone to take at least even short fasts (at least 5 days) since the health benefits are so obvious. The research shows that intermittent fasting is also a great thing to do.
Most people think that they will die with fasting, and it truth, if you don't know what you are doing you could damage yourself.
The keys to a successful fast are lots of good, clean, pure water and learning to listen to the signals of your body. Take it easy and trust me, it will change your life.