I think the best method for cutting weight for a 24-hour weigh-in is to get your weight down and back up as fast as possible. The less time your weight is down the less it will affect your performance.
I'm assuming your weigh-in is on Friday morning at 9 am or so. On Thursday after lunch, stop all eating and drinking. If you are really hurting you can have a piece of toast with peanut butter on it in the evening but absolutely no liquids. Basically, I sweat all of the water weight out. How you do that really doesn't matter, but the important thing to remember is to expend as little energy as possible. Don’t do any running or calisthenics like a high school wrestler would do. You can use a steam sauna, dry sauna, hot baths, and even a car heater all bundled up in sweat clothes if nothing else is available.
As soon as the weigh-in is over, you should rehydrate and gain your weight back as fast as possible. I bring my cooler packed with food and liquids with me to weigh-in. The second I step off of the scale, I chug about 1/2 gallon of Gatorade and begin eating immediately. You must gain all of the weight back that you lost or your strength will be affected. As long as you gain all of your weight back or more than you lost, your strength will be 100 percent or sometimes even better.
For an example, this is exactly what I did for the Arnold weigh-in on Thursday at 9:30 am.
Wednesday, 12:00 pm, 240 lbs:
After training, I had a protein shake and then stopped food intake. My body weight was 240 lbs. I did several hour long hot baths with my body completely submersed under the water except for my nose and mouth. And when I say hot, I mean as hot as you can possibly tolerate without scalding yourself. With that, a dump, and urinating a couple of times, I was down to 232 lbs by 6 pm.
Wednesday, 8:00 pm, 232 lbs:
I had one piece of toast with peanut butter. Then I put on five layers of sweat clothes, including a hooded sweatshirt, and drove from Lansing, Michigan to Columbus, Ohio. I put the heat on full blast and pointed it directly in my face with the hood drawstrings pulled as tight as I could while still allowing me enough vision to drive. Four and half hours, later I arrived in Columbus seven pounds lighter.
Wednesday, midnight, 225 lbs:
After dropping all of my stuff off at my buddy's house just north of Columbus, I drove to a nearby hotel to use their dry sauna. After a fifteen minute argument with the night manager, she finally relented and let me into the sauna. At this point, I was already feeling really dehydrated, and it was becoming difficult to get a sweat going. I spent three and half miserable hours in the dry sauna with short 2–3 minute breaks every half hour or so (where I would go to the bathroom and try to squeeze out anything I could). This netted four more pounds and left me feeling half dead.
Thursday, 3:30 am, 221 lbs:
I went back to my buddy's house and tried unsuccessfully to get a couple hours of sleep. At 6 am, I spent another hour in a hot bath and weighed 219–220 lbs on my scale.
Thursday, 8 am, 220 lbs:
I drove out to the meet site to check my weight on the meet scales. Fortunately for me, I was almost one pound under. If possible, try to coordinate your scale with the meet scale ahead of time since we all know several pound differences between scales are quite common and the meet scale is the only one that matters.
Thursday, 9:30 am, 219.6 lbs:
I weighed in at 219.6 lbs and immediately began drinking and eating. I continued to force feed myself all day and never went more than an hour without eating or drinking.
Thursday, 6 pm, 238 lbs:
I drank two gallons of Gatorade and one gallon of skim milk during the day and ate copious amounts of food, which allowed me to gain almost 20 lbs in around eight hours. This is the key. You must be disciplined and relentless in your rehydration and eating. If you slack off here it will hurt your performance. I stopped checking my weight after this because I didn't want to play head games with myself before I lifted, but I am certain I weighed 240–245 when I stepped on the platform Saturday to lift.
So all in all, I lost just over 20 lbs in about 20 hours and then put it all back on in around 10 hours. Again, the quicker you get down and the quicker you get back up, the less it will affect your strength. I know many guys who use diuretics to get down and IVs to rehydrate. I personally don't feel that either is necessary, and it is taking the easy way out.
Potassium-wasting diuretics can get you into big trouble if over done due to potassium depletion. Without a blood draw, there is no way to know what your levels are. Since taking too much potassium can be just as deadly as not enough (it will affect your heart rhythm) and they are famous for sapping strength, I avoid them. While IV fluids can be an effective way to rehydrate, you had better have someone who knows what they're doing when administering it to you or problems can ensue there as well. To me, these risks aren't necessary and are just the easy way out.
One word of caution—this is what I do and it works for me. Everyone has a different tolerance for heat and dehydration, and messing up here can have dire consequences. This is just for educational purposes only. What you do is completely up to you and at your own risk. Good luck!