I get occasional headaches from dehydration. As I rehydrate it goes away. Don't drink alcohol, do drink water all day, normal blood pressure. Food?

No caffeine. Stay away from caffeine containing products as they cause you to urinate and dehydrate you as well. Stay away from tea, coffee, dark chocolate and other caffeine containing drinks.
Maybe You Need Salt. If you're frequently dehydrated, try drinking an electrolyte drink like gatorade or g2 which also contains sodium or salt instead of water. Of course, salt should be used in moderation.