Is there are a guide published by microsoft or somebody else about the best practices for creating scalable web applications? like patterns to use and how to do data access.
The Microsoft Patterns & Practices group is a good one stop shopping ground for that.
Edit: Here is their guide specifically on Scalability and Performance. Chapter 6 includes information specific to ASP.NET Performance and Chapter 17 includes information about Tuning.
The best advice I can give is to do these three things (roughly in order):
- Avoid unnecessary postbacks
- Avoid excessive viewstate
- Spend your time optimizing your database
This is an excellent article on the subject of scale: http://msdn.microsoft.com/en-us/library/bb924375.aspx Which basically discusses the issues of load balancing, session affinity and caching all of which become important in any scaling discussion. If you have a specific question after that, let us know.
I'd take a read through the MVC Storefront series. It's ASP.NET MVC based, but demonstrates a nice approach for creating a loosely-coupled, well architected website. You could easily apply most of the principles to a webforms site as necessary (though I'd recommend going with MVC if yo have the choice...)