What happens, is when you diet you lose both fat and lean tissue, at rates up to 50% each. Since a pound of muscle requires 50 calories a day to keep it functional, the loss of muscle through dieting is slowing down your metabolism.

For example, if you lose 100 lbs, and of that 65% was fat (65 lbs) and 35% muscle (35 lbs), you have reduced your maintenance calorie level by 1750 calories.

Couple this with the fact that you are probably eating the same amount of calories as you did 100 lbs heavier, there is less of a difference in the energy equation Energy in (calories) = Energy out (maintenance + exercise)

The answer to solving these problems is adding muscle back, ie lifting weights. I learned this the last time I yo-yo'ed back from 308 to 410. I wrecked my metabolism from losing so much muscle. So this time, I started weight training like a demon. As a result, my rate of loss only decreased 0.26 lbs per week (1.36 lbs to 1.10 lbs) from 2002 (49 lbs lost in 36 weeks) to 2003 (57 lbs lost in 52 weeks).