Not that I know a lot on this subject, but to add my 2c worth...
I think it's partially genetic - Ie, some people will be predisposed to sagging skin and some others will be luckier in that regard. Just like most people have varying amounts of stetch marks and such. (I know one girl that didn't get a single mark from her pregnancy - cow
It also depends how much you lose per week - and I would *highly, highly* recommend keeping it realistic. Another friend of mine lost a lot of weight when she was 16-ish (about 20kg in 4 months or something ridiculous along those lines) and know shes stuck with the saggy belly, even though she has put on the weight and lost it several times since then.
Also, congrautlaions again Shalimar, you must be so proud!!
And good luck to the o2bthin