From my reading online one of the best things you can do is weight training. There was a study of two groups of obese people who were given 800 calorie diet, one group was given cardio 4 days a week and another weight training. Don't remember how many weeks they did it for, but at the end the cardio group lost 37lbs, 10 of which was muscle and the weight training group lost 32lbs of which they didn't lose any muscle. Building/maintaining muscle will help prevent some of sagging skin. Also reducing your body fat percentage composition and replacing with lean body mass will help with the appearance of the skin and your body. This is why it's so important to get adequate nutrition, water and protein intake while dieting and even more so when weight training as your body needs proper nutrition to building new muscle and recover from workouts. The more I read and learn the more I discover how much more important weight training is than cardio. Although cardio should still be included as it helps strengthen your cardiovascular strength.

Also it's best to do cardio AFTER weight training, weight training sets up your body to lose more fat, so doing cardio after tricks your body into burning more fat. I have a sorta fancy heart rate monitor that gives me extra info, before I used to cardio before weight training to warm up but my overall workout would only burn 15-18% of fat, after I reversed it I got it up to 30% of fat. When I do weight training that percentage is always higher than cardio. Burning calories is great but you want to burn more of that fat to get leaner definition and IMO it's less wasted energy when you burn more fat.

The slower you lose the better your body will recover as your skin shrinks. Our skins is capable of stretching quite a bit and recovering but the best way to achieve that is with slower weight loss...giving your body time to catch up with your weight loss.

Some of other things I read that you can do is dry brushing-which you shouldn't do more once a week as it causes micro abrasions on your skin which can cause micro infections. You should also brush softly, over time this is supposed to stimulate your collagen (supposedly). Make sure you moisturize your skin with things like coconut oil-the more natural the better. There are also wet brushing gloves you can get to use while taking a shower. I don't know how effective these will be but removing dead skin will help lighten stretch marks and improve appearance of the skin. Just don't over do it as you could damage your skin. Moisturizing your skin will help it recover better from weight loss and any damage. I prefer organic coconut oil, you don't need much to cover a lot (it is also great for moisturizing/growing out hair, guarding hair against peroxide damage when dying/bleaching hair, it even thicken my eyebrow hair, it's amazing on any wounds you may get as it's anti microbial (I get butt pimples during summer from fabric irritation but coconut oil makes them go away); it's also wonderful for your digestive track to kill bad bacteria (but follow with spoon of organic honey to reintroduce back the healthy bacteria)...anyway its uses are endless!).

I have also read doing cold by hot water showers (or it's reverse) helps stimulate your skin as well although I haven't tried as that sounds like torture to me. So that's it for now...