This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
Sweat (Sam Henry firstname.lastname@example.org)
It's hard to say what to do without knowing what you do now. None of us can
replace as much as we lose while we are losing it. The trick is to keep
from going into deficit.
Do you hydrate yourself every day, all day long? Min 2 qts/day.
Do you hydrate yourself extra before the ride (like a qt an hour for 2 hrs
or so before the start).
Do you use sports drinks to help with trace element losses? I use Exceed at
25% solution for the 1st half of long rides, orange juice at 25% for med
rides, and plain water for short rides.
What is your consumption rate during rides? I start drinking 30 mins into
the ride and drink a qt an hour whether I am thirsty or not. If you are
thirsty, it is probably getting pretty close to too late.
Do you eat while you ride? Things like bananas, oranges, and pears provide
fuel *and* coolant, along with some nifty minerals and such that your body
needs to make the cooling system work right. I eat fig newtons and such
right as I start and eat every 20-30 mins after the first hour. Pears,
particularly, are an easy-to-eat thirst slacker.
What kind of hydration regimen do you use *afterwards*? I immediately start
drinking at the end of a ride, starting with a quart of water followed by a
quart of full-strength sports drink (Exceed for me). I also find something
to eat that is high in complex carbohydrates. All this within the *first
hour* after the workout. The eating and drinking are intertwined. Then I
drink another quart of something that sounds appealing. Then I go back to
my drinking all day long to get my "normal" two quarts.
I might have thought I would slosh, but I never have. And most of my riding
is done at temps above 80 degs and in high humidity. If you are urinating
infrequently and the urine is a dark color, you are underhydrated, whether
you have exercised or not. No matter how much you sweat.