Expanding my ASP.NET solution to multiple sites ensuring maintainability

I have a web site written in ASP.NET WebForms. I need to distribute the web site platform i have develop开发者_StackOverflow社区et to another website - and this looks like i may have to do this a couple of times more.

Each website is running on its own SQL Server database and have a different layout. But each website shares codebase with the other sites.

Im looking for a way to lower the maintainability of these sites when i make changes or new features to the "main codebase". My concerns are how to update, preserving the layout (themes?) with a minimum of effort and (in a perfect world) choosing only which sites should get a particular upgrade. (eg. a bread crumb) - and keeping track of these.

Any feedback is appreciated

I would suggest you to learn multi tiered architecture design for your web application.
There are many cool books and ASP.NET 3.5 Application Architecture and Design is one of them.