Blood drive at school

I'm signed up for a blood drive at my hs tomorrow. Is there anything I should do specifically in terms of what I eat/drink? Last time I did one they said they couldn't find a blood vein or some ****, which was disappointing.

You need to be well hydrated. Like really well hydrated. Your veins sink back into your body when you're dehydrated and that's probably why they couldn't find one last time.
DRINK LOTS OF WATER

period blood is gud