Thank you for responding to the questions.
The best way to prevent saggy skin is to lose weight sensibly. Loosing weight too quickly will increase your risk of saggy skin. The skin needs time to adjust, so slow, gradual weight loss is preferred. 2-3 pounds a week is a healthy, sensible number to keep in mind. I know that it's difficult to imagine that 100 pounds will come off 2-3 pounds at a time, but it will. Stay positive. Starting off with 1500-1800 calories may be sensible at this point. Your diet will be very important. It needs to be well balance. When you do eat, opt for whole grains and lean proteins. A healthy diet, particularly, lean proteins will help your skin and muscle development. Also, you can consider taking a multivitamin. Most diets are lacking in essential vitamins and minerals. A dieter can easily be at an increased risk for vitamin and mineral deficiency. A complex multivitamin will also help your skin stay healthy. Stay hydrated. Our bodies are mainly composed of water. Don't skip the water. 64 ounces a day is a good goal. Some people recommend more; however, if you're not much of a water drinker, it's a good starting point. Exercise too will help tone muscle. Your skin will look better draped over some muscle tone. When you exercise, you may lean toward more cardio, for the weight loss benefit, but don't forget the weight training too. It is important. If you don't exercise much, start with simple goals, like 5-10 minutes of walking and add more time every few days. There is no guarantee that you won't have saggy skin; however, you can do your best to prevent it. Also, based on the 1988 in your username, if you are in your early 20s, young age helps too. The body tends to bounce back better when a person is useful.
This will take some work, but remember not to get overwhelmed. Start with small goals of 2-5 pounds at a time. You can do this. Also, when you do have a bad day (there is no such thing as a perfect dieter), don't beat yourself up for it. One thing I tell patients is to look at their week. Maybe they'll have 6 good diet days and one day when they didn't do as well. That's fine. The good days add up, but no diet or dieter is ever perfect. Don't throw in the towel over one bad day. Just try to make a commitment to having a good day tomorrow and stick with it.
I hope this helps.