The Joker is a liar.
The impression I got was that the Joker doesn't have endless, multi-layered plans. Most of it is improvisation which he later claims to have been pre-planned in order to mess with others' heads.
And I don't think the Joker is a liar (most of the time). I think he's the sort of man who, most of the time, is telling the truth... at the time that he says it.
There's no way he could've what he did without alot of preplanning, including inplanting that exploding cell phone, setiing up road blocks and setting the explosives on the two ferries. And yes he does lie thoughout the movie.
Certainly the plot to try to capture Dent was set up with pre-planning -- I think he knew that Dent wasn't Batman and was explicitly hoping to either kill Batman or get himself captured so as to force Batman to choose either Dent or Rachel to save.
Most of his actions, though, I think were improvised. And I think the only lie he told in the film was the lie about where Rachel and Harvey were.
(He tells conflicting stories of his origins, but I don't think those are lies. There's a line in Batman: The Killing Joke
by Alan Moore where the Joker says that he remembers his past differently at different times. "Sometimes it's this way, sometimes it's that way.... If I have to have a past, I'd rather it be multiple choice!" I choose to interpret his stories about his past as being the absolute truth... as he remembers it at that time.)