Headphones on as I board, politely taking them out as I present my BP to the FA, then straight back in as I take my seat, and up until the announcement to switch off all electronic devices. This is a pretty clear indication to the person sitting next to me that I'm generally not into meaningless chit chat. As above obviously a polite hello/nod/etc in return is standard.
In saying that, occasionally I've sat next to some really interesting people and have had quite in depth chats, however this is generally not the norm.
