I am building an internal, data-heavy website for my company on the Microsoft stack (ASP.NET 2.0/C#/SQL Server). Lately I have been trying to improve the layout and design of this site, partly because I want it to look good for my employers, but mostly because I want to learn about page layout and design.
My question is, what is the first thing I should know about improving the layout and design of pages in a data-intensive (e.g. lots of large grids, validation, minimal navigation, etc.) website. I really have no idea where to begin.