I think if we do this, we need to completely rethink the prequels, pretend they DO. NOT. EXIST. - then start from scratch.
They need to be designed to set the audience up for the OT, while still preserving the surprises of the OT. The fate of Anakin and the origin of Darth Vader both need to be ambiguous enough that the audience can accept Obi-Wan's story in ANH, if perhaps they are suspicious of it. Likewise, the Leia-as-Luke's-sister aspect needs to be ambiguous, as well. Hell, maybe the birth of the twins should occur off-screen, or between "Episode III" and Episode IV. Yoda may be one exception to this - he kind of needs to play a part, and if he does the surprise of his identity in ESB can't be preserved.
The Jedi should be treated the way they were in the prequels. I.E., they should be portrayed in such a way that their powers are more spiritual in nature and less well-known to the general public, leaving enough people skeptical so comments like "That anicent religion ... " and Han's disbelief in the Force make sense. The Jedi shouldn't have such a prominent role in society and government - they should be on a different planet than Coruscant (if we use Coruscant) and should perform a more religious function, and should only aid the Republic under secrecy, and only if abstaining from involvement will cause a dangerous imbalance in the Force (which should also remain mystical and spiritual, none of that "midichlorian" bullshit).
Adding in characters just because people like them (Boba Fett, Threepio, Artoo, Chewie) is a HUGE mistake and should be avoided at all costs. Characters should only be used if they are essential to the story.
The biggest challenge in doing this is really trying to forget about the prequels as they exist in order to make something entirely new that isn't just, as Adywan likes to say, "fanwank."