Thank you for donating.
My children have needed blood donations as well and one of those children is O-.
Besides hydration and finding a better trained center (which is something , one thing you can do AFTER the donation to help prevent bruising is to put firm pressure on the site for 5 minutes immediately after they remove the needle. It seems so counterintuitive that firm pressure prevents bruising, but I have seen it work dozens of times for my kids.
I'm not even sure why, I just know that a nurse told us once, we did it, and we were amazed. Now, if they fiddle with the needle to find a vein, sometimes that bruises no matter what, but mostly, pressure helps prevent bruising and pain afterwords.