depends on how you kill yourself, you might want to stab yourself in a major vein.. but i dont know if you have 20lbs of blood though.. and i dont think you'll rot off 20lbs in 8 days..
You could cut your head off and lose 20 pounds of ugly fat that way. Cut you calories back big time in the next few days and do a lot of cardio, the day before the fight cut back on the water and get in the sauna and sweat the rest off.
if you're a lightweight forget it and pull out. If you're big just do what lukeluster said. Only thing you may get to your fight a bit drained. The weigh in is 24hrs before right? And yeah, if you've never done it before you may have wanted to start from fighting a little closer to your normal weight and then gradually cut to the lower weights. It's a matter of learning how your body reacts and also pushing it a little futher every the time. Otherwise you may just panic half way there.
Go on a crash diet for a couple of days. No dairy products at all. Try to stay away from Carbs too. Stick with Fruit, Veggies and Protein. You should lose about 5-10 in a week. Still drink lost of water. Then the day before start cutting, try for at least 60% off of what you need to lose. Go to bed with no drinks or anything and cut the rest in the morning.
Don't do it. 20 lbs in 8 days is just too much. How much do you weigh? Height? Body composition? And why do you need to cut the weight? *edit: apparently this is for a fight that already happened?
beyond using an oral saline laxative to empty your GI tract, you can do it with water weight...sauna, diuretic. Although it can make you VERY weak and VERY thirsty and half dead. someone told me you can safely lose 5% of your hydrated body weight in water weight before it would be unsafe. it all depends on how much you weigh to begin with. 300lbs is easier than 150lbs of course.