css css and more css!
your asp.net web sites should be split into 4 areas - the aspx page with just html (just what needs to be in the browser), the aspx.vb or cs code behind file with all the server processing, a folder full of css files (for presntation) and a folder full of script files (for javascript) if you use them.
Dont mix function with form (i.e dont put layout, colors and looks in the aspx/html page). This not only makes it easier to read and manage when you are developing but also easy to change things later.
As long as you organize everything correctly (personally i use divs) then you can place it where you like. And you can do some crazy things. Have a look at the home page on a site i built recently for a customer
http://www.inspirationsbuyer.com which is all done using divs and css (with the odd javascript thrown in). Hardly anything is in a horizontal direction and took very little time to create.
CSS is VERY powerful when you know all the things you have to your advantage.
Also have a look at AJAX as this is a powerful tool and is free (the microsoft version is) and make sure you get the AJAX extensions and control toolkit ctp's. We are now building websites with the same rich ui as windows forms because of the power ajax gives us.
Both of these require time to learn and plenty of practice but will make site development much faster and easier to maintain and you will soon build your own library of tools.