I think we're belittling the TG cause (if there's such a thing) by including everyone who wears clothing commonly associated with the opposite sex. I don't consider people who cross-dress for practical reasons (e.g. actors who portray a certain character, people who wear something of their partner because it's the only clean/warm thing around the house) transgender and one could argue that you're only a cross-dresser in the strictest sense (and the subject of this website) if you cross-dress to fulfil a psychological need.
In my opinion, any psychological need or want that a person has that can be fulfilled by crossing gender boundaries (e.g. presenting as the opposite sex), must be the result of some psychological gender-related incongruence and that makes that person technically transgender. Additionally, people who get a sense of normalcy out of cross-dressing or who get turned on by it aren't the 'norm', and that's what makes them transgender as well. They're not cisgender, therefore transgender.