Personally, I admire Beats by Dr Dre because they have a good piece of business, and also I have thought about how you would go at marketing headphones and I really come up with nothing unless I would be famous or have a person of authority with me when it comes to sound.
They market them by getting famous people to wear them. Firstly Dre will get all his artists to wear them in public and in their videos likely for free (Eminem 50 Cent etc), then they will give them out to other artists for free in the hope they wear them out in public. Then they pay other artists and athletes to wear them and that has a knock on effect as it's seen as cool and the new brand to have and it spirals from there. I remember seeing a video of the Manchester United squad getting off their coach before a game and pretty much every other player had a pair or beats headphones around their neck. Just look at all the pop videos too. From Jessie J to Nicki Minaj they always feature beats headphones or products prominently.