I would do some trial runs in a sauna. Go in there and see how much water weight you drop in a given period of time. Start lowering your carbs when the fight approaches and up your water intake.
Do you weigh in during the morning or evening?
Let's say it is in the morning to give an example. Cut out any fluids several hours before bedtime the day before. During that time do some moderate to slow cardio. Then go in the sauna and and sweat it all out. Weigh yourself every so often to check your progress. You should lose some more weight during your sleep. Wake up in the morning. If you are still off, go back and visit the sauna. On your way to weigh in, start spitting into a cup during the drive.
After you weigh in, start drinking pedialite to rehydrate yourself. You darn well had better be hydrated fully before your fight (the fluid helps protect the brain).