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I started fasting on Yom Kippur as an adult, about 10 years ago.
A couple times I had to drink a glass of water either after Kol Nidre or before Shacharit, because I felt completely dehydrated (I did go on with the fast after that)
What steps do you recommend to avoid this situation? Foods to avoid during the previous meal? Specific hydration steps?
Please see "The Segal Guide to Fasting For Yom Kippur (from a Medical Perspective)," written by a physician. The very first point he deals with is the thirst issue you raised.
Most people think the difficulty about fasting is feeling "hungry". However, avoiding thirst is much more important for how you feel. Not only do you avoid the discomfort of thirst but you are also well hydrated and swallow frequently, so your stomach does not feel as empty.
One important way to remain well hydrated is to avoid drinks or foods that cause your body to get rid of water. Such foods and drinks include alcohol, tea, caffeinated coffee and chocolate. Another important rule is to avoid consuming much salt. Salt causes a person to feel thirsty despite having a "normal" amount of water, because extra water is needed for the extra salt. For this reason you should avoid processed foods containing lots of salt such as pickles, cold cuts, or cheese. Most tomato sauces, canned fish and smoked fish have a lot of added salt. Since Kosher meat has a high salt content it may be best to choose a main course such as fresh fish, canned no-salt tuna fish or a de-salted meat such as boiled chicken.
By avoiding these types of foods and drinks in the several hours before a fast, you can avoid either losing water or needing extra water. Other actions that cause the body to lose water, such as perspiring in warm clothing, should also be avoided during the fast.
Hope you have a successful and meaningful fast this year!
From a personal perspective, I have found that the easiest fasts I have had, came when Yom Kippur was on a Monday, and Sunday morning I did a 3-4 hour run (training for a fall marathon).
My theory is that knowing how dumb an idea it is to do a 3-4 hour run, mere hours before a 25 hour fast, I try to compensate by drinking the rest of the day, every 10-20 minutes having a mouthful or two of water at time.
This is actually how I try to optimize hydration during long distance races, but doing it before Yom Kippur seems to have helped.
Based on a comment from Monica Cellio I feel I should add that the reasoning behind hydrating in the described fashion is based on the uptake rate of the typical persons gastrointestinal tract is about 800 mL to 1 litre an hour. So drinking more than that at once means your kidneys will just discard it.
For your stomach to effectively absorb water you want to keep it constantly absorbing, with a small amount of physical pressure. A couple of mouthfuls at a time keeps the stomach somewhat occupied, and the physical presence helps generate a very small amount of pressure that helps the absorption process.
Thus doing this every 10 minutes or so keeps the pressure up, and maximizes the time your body has to absorb the water.
Of course expect to visit the potty a lot as a consequence of this!
The key to not getting dehydrated while fasting is to thoroughly hydrate in the days leading up to it. If you drink a lot of caffeine (or alcohol) especially, gradually back off from that and replace it with water. I start the day after Rosh Hashana, replacing other drinks with water or fruit juice over the course of the week. In the last 24 hours before the fast I drink no caffeine at all, and I drink plenty of water -- a couple quarts over the course of the day. Supplementing this with watery foods (like fruits) is also good. (I'm focusing here on the drinking aspects; there are other questions on this site about foods to eat or avoid eating.)
You'll still be hungry, and thirsty. You may well get a caffeine headache. But if I do this I don't feel parched and desperate to drink something.
If you still feel like you need to drink some water, ask your rabbi if it would be ok to swish some water around in your mouth and then spit it out instead of swallowing. Depending on your situation he might be able to offer you that leniency, and that might be enough to trick your body so the feeling abates.
This is all assuming that there is no health issue, of course. If you have something special going on, consult your rabbi and your physician before Yom Kippur to find out what you need to do.
I heard a Segula from Baba Sale Zatsal (R. Yaakov Abouah'sera) but I never tried it :
Drink a glass of water in front of the Mezuzah of the room.
My strategy, which has worked well for me in the past, is to drink 6 (six) liter-size bottles of Glaceau SmartWater on the day before the fast.
The reason SmartWater works (much) better than water is that it contains electrolytes to replace those that would otherwise be flushed out of your body by drinking tons of water. Your body needs to have them, or else you will start feeling faint, winded, and generally tired and strung-out. The extra water consumption that is necessary and desirable before a fast can cause this problem unless electrolytes are added.
It's very hard to make yourself finish the six bottles, but you should; that amount is necessary if you want to ward off thirst for an entire fast day.
Don't eat heavy, salty, sugary, fatty, carby stuff before the fast, but do eat a huge meal of protein, fiber, fruits, vegetables, and other nutritious goods.
As long as I am not taking special OTC or prescription medications during the fast, which can mess things up, I am always comfortable on fast days because I use this strategy.
