The 'art' of modifying is to enhance your car without making it look like it crashed into a branch of Halfords. IMO a well modified car will make people to a double take, because they think "hey, that one's different, isn't it... or is it?".
But with such things 19" wheels and R-line bumpers available from the factory, it's becoming more difficult to be individual and remain subtle. So apart from a spot of lowering and perhaps some BBS wheels, I don't think I'll be doing much to my next Golf.