What could I possibly do to help prevent lots of excess hanging skin after bariatric surgery?

Not a lot. One of the good things about bariatric surgery is that it leads to rapid weight loss. Unfortunately, this also leads to poor retraction of the skin. People who tend to lose weight more slowly through diet and exercise tend to have better skin retraction, but it is by no means perfect. It's better to think of bariatric surgery as a means of treating all of your other health issues.
Hard to prevent. Excess skin is hard to really prevent. When we see a patient who has lost 100 pounds, we see healthy person no longer suffering from joint, lung, diabetes, and blood pressure problems. Better to be healthy with alittle skin. About 20% of our patients eventually get skin cut off. Insurance covers some.
Exercise. The more that you tone and tighten the muscles underneath your skin, the chances are that the skin will become more tone as well. So hit the gym as much as possible to try to prevent the saggy skin.