Foremost are the movies. And I'm strict by the movies. It's what we actually see on the films not what someone else has interpreted for us. That way, it allows personal GM interpretation instead.
I'm thus allowed to have my desert Stormtroopers be just regular Stormtroopers in hot weather gear and not some elite corps, for example.
The EU is love hate for me. Some of it I love, some of it I hate. Thus, I cherry pick it. My players have never been EU encylopediacs so I get to slip in what I like and sometimes I later have to explain it wasn't actually my creation.