A lot of it is out of our control to be honest, it's down to things like age and how elastic our skin is.
I think there are a few things you can do to help yourself. Weight lifting is great for shaping muscle and toning up. Don't be put off and be afraid that you will bulk up, unless you are a man and eating at a calorie surpluss it won't happen (or taking drugs!). Yes you can retain water from sore muscles but weight lifting gives great shape and help retains muscle as you lose weight, and the more muscle you have, the more calories you burn when resting
Keep yourself and your skin well hydrated. Drink plenty of water. I know the recommended is around 2 litres but it does very a bit for people. The best way to check for dehydration is colour of your wee! If it's dark then up the water, if it's very pale straw colour then you are bang on target!
Do dry body brushing to exfoliate and use a good moisuturiser designed to help tighted skin up.
And probably the best way to avoid excess skin if at all possible is lose the weight in a sensible way, lots of healthy foods, don't starve yourself and lose the weight at a steady rate not too quick (sounds like slimming world to me!!).
I know it would be upsetting to put all that hard work in and still not feel happy in your own skin but at least health wise you would have reduced the risk of heart disease, diabetes and all other kinds of things and in your clothes you will look amazing!