Improve speed performances in C#

This is really two questions, but they are so similar, and to keep it simple, I figured I'd just roll them together:

• Firstly: Given an established C# project, what are some decent ways to speed it up beyond just plain in-code optimization?

• Secondly: When writing a program from scratch in C#, what are some good ways to greatly improve performance?

Please stay away from general optimization techniques unless they are C# specific.

This has previously been asked for Python, Perl, and Java.