This question was very helpful, however I have a list control in my report, and when the report grows over 1 page - data in the header only shows up on the last page of the report.
Apparently, hidden textboxes have to be on every page of the report for header to function properly, how do i do that? The only control i have in the list is a textbox with bunch of text that grows way over 1 page.
Solution: All I needed to do is to use Report Parameters with queried values from my dataset; and then reference
=Parameters!Name.Value
in the textbox in the header of the report.