As per the sticky

Yeah, but surely if I put on a suit I'll end up in F?
Hardly. The oft-cited garbage 'rules' of dressing nicely and putting on a smile will not work. Although it naturally never hurts to be prepared to sit in a premium cabin and be polite to the check-in staff as always. Many anecdotal tales from frequent BA travellers here will attest to being upgraded to J or even F whilst unshaved and decked out in scruffy jeans.

Lots to be said about the rest of the post too, but its all in the stickies !