You haven't exactly been inundated with tips and I'm afraid I haven't got any wisdom to impart either. I guess we'd all like to know the answer to this one.
My two used to do much, much more around the house when they were younger. They used to have a few regular chores and I also used to ask them to extra one-off things. They used to do it all cheerfully. I think they enjoyed helping actually - it was, perhaps, bit of a novelty or a game.
Over the last couple of years (they're 18 and 16) they've got a bit messier and more absent-minded about domestic stuff and I've often cut them some slack because:
(a) they've often got something important (exams, coursework, music exams, uni admissions stuff etc etc) going on and they've got a lot of pressure on, time-wise
(b) because there are so, so many other potential arguments with teenagers (setting boundaries about going out/ coming in etc, you sometimes have to choose your battles and let go of some of the smaller stuff
(c) because I just get sick of wasting my breath - I can tell them every day to do something but they'd still forget to do it unless I tell them again. I get so fed up of nagging - open your curtains, bring that mug downstairs, hang those clothes up, put your bowl in the dishwasher, bring your laundry down - God, it's tedious.
I despair really. I make them change their beds once a week but they don't do much else other than join in emptying the dishwasher if they're around. Their rooms are not as bad as some of my friends kids - they do give them a blitz once in a while - but they've definitely slipped back a long way from how they were when they were younger more enthusiastic kids.
Sorry - not much help, really! How old is your teenager and what do you want him/ her to do?