Well you have finally reached your goal! You lost XXX amount of pounds in XXX amounts of weeks. Sure, you feel better, and even look better. However, since you lost weight (note I didn't say bodyfat), you have a noticeable amount of loose skin.
Let's start from the beginning....
What is exactly is loose skin?
When the body goes through a rapid weight loss transformation that outer layers (your skin) needs to time to catch up. While you were getting larger, your skin was taking it's time to enlarge too. If you notice you can see larger people with stretch marks. Hell, I still have my stretch marks from when I was larger.
You can start off with waiting... Your going to have to give your body time to adapt to it's new surroundings. If you're thinking surgery, don't. It's been generally accepted by surgeons to wait at least a minimum of two years before even exploring the option.
How can I prevent this in future?
You can develop a plan that is not based around losing weight. Your focus should be the reduction of body fat. I was able to keep my loose skin to low amounts, because I lifted weights from the beginning. Thermogenesis is your body "burning" fat cells as fuel. Your body uses the stored fuel and the cell shrinks. It's like an inflated balloon. You release the air out, the balloon shrinks. When all the air is out the balloon is deflated, smaller but still there.
Muscle at the most basic understanding moves into that space where the reduced fat cells lie. You may have heard that muscle turns to fat. This is not true. Muscle and fat are different types of cells.
Lift heavy and try not to focus on pure weight loss. If you have a large amount of body fat to lose, well expect some of it to carry over. Just give it some time and your body will adapt to it's new shell.
*http://www.ehow.com/how_2135049_loose-skin-after-weight-loss.html