Good article on making ASP.NET applications perform well
Came across this article recently (someone pointed it out to me but I can't remember who, sorry). It has 15 timeless tips for ensuring your ASP.NET applications perform well.
Talking about performance, if you are looking for a tool to test your application's performance and scalability, we just released an update to WCat. There is also an x64 version available. (I've added a link to WCat 6.3 to my load test tools page).