Ok so lets take Shane Mosely as an example he walks around at 160+ so by the time he is in camp he is a strict diet and when I say diet he wont be eating any unecessary fatty or sugary foods. Combine that with training 3 times per day 6 days per week he will drop ALOT of weight so he may well get down to 150Lbs by doing that alone. The last few Lbs are the most difficult to lose, so you will probably have to start fasting a day or two before the weigh in and that will take off about a 1Lb and then maybe you might have to run in a sweet suit of go in a suana. Its really really difficult to cut weight for some people (like me)
It's not about dieting, it's about cutting water, you train/diet down to about 10lbs over your target weight (some go 20-25lbs though). Then you load up on water the weak before the fight drinking 3 galons a day. Then the day before the weigh in you stop drinking water entirely and only suck ice cubes, you take diuretics and get yourself in a steam room. Once your within 2lbs of the target weight you goto sleep, by the morning you should be at the weight limit. Then you use rehydration fluids/electrolytes to gain the water back without being drained and you weigh in at say 10+ lbs over the weight limit in the ring