A lot of people tend to gain weight after they have lost a fair amount - a natual boon to the diet industry turning them into a perpetual economic motion machine of sorts. The metabolism appears to lower and tends to stay lower. Even exercise doesn't do much (sadly) - although I find it a necessary component and it is certainly a healthy thing to do.
Researchers at the Pennington Biomedical Research Center have a new weight loss calculator that takes some of this shift into effect. It won't be perfect, but it should give a better idea of how a diet will respond. (note it requires Java - I normally have that turned off as it is a security risk.)