(This is a quick check-list from my experience. May turn into an illustrated slideshow when I have time)
And here is a real life dissection of how I tuned a Haskell program: In Search of Performance in Haskell
This has always been a mysterous subject — a lot of people talking about it but not many people demonstrating it. Basically, what you want to do when reading Core is to make sure the checklist above has been realized by the compiler. For example