I was actually almost hoping for the hair loss, as it would have given me an excuse to shave my head for a while. While my hair is now not quite as coarse as it used to be and much thinner (the hair itself, not the volume), I haven't experienced any hair loss at just over 6 months out.
That said, I've been beyond religious when it comes to making sure I have my Protein and fluids each and every day.
As for the dumping - as already mentioned, avoid the sugars and fats and chew your tiny bites as much as humanly possible. I have only dumped once and it was by eating regular ketchup (I now use reduced sugar). It was a bad day (~6 hours), but definitely not the end of the world. After that I learned to try all new foods while at home and to ensure anything I try is extremely low in sugar (I've avoided fatty foods at all costs, so no experience there).
The sagging skin is now more of a badge of honor. While it looks terrible in the shower, I'll happily deal with it knowing that I can be active, healthy and truly get back to living life instead of just surviving.
Unfortunately I don't know that there is anything you can do to combat any of these 3 issues pre-surgery, but in my opinion none are worth fretting about.