Keys to a Successful Complaint Letter

[octopi]

I recently received some subpar service at a local grocery store, and wish to make my displeasure known. After working in retail for many years, especially in a position to read a lot of 'complaint' letters, I have only come to realize two things:

1- What not to write (how much money you spend there, how the person in question should be fired, free stuff etc etc)
2- No one actually cares about these on the store level

So what are my best options? To write the store and head office? Is this just a waste of time? Do you write many angry letters?