Warner Brothers have released several short videos as teasers in advance of The Hobbit: The Desolation of Smaug preview video coming out on March 24. The initial video release will only be available with the Blu-Ray and DVD, together with the online showing that you can access after purchasing these. You should not have to wait long after this before the trailer goes out on general release, though.
In any case, here are the shorts, one of which gives a sneak peek at Smaug:
Bilbo as a hero:
Thorin’s quest in this one:
Beginnings in this:
And finally, a return to Middle Earth: