In 2000, GWB helped himself immensely by choosing Dick Cheney as his running mate. Seen as being a lightweight, Bush needed "gravitas" as the mainstream media parroted for days.
On the brink of announcing his veep pick, I think Obama is in the same boat. He needs someone with some weight, some heft, some inside Washington know-how. For all of his talk of changing Washington, Obama has simply not been an agent of change while in Senate. He needs a Vernon Jordan-esque person to help him wade the political waters around the Potomic.
With that in mind, the best choices for Obama, in my estimation, are
Former Senator Sam Nunn
Senator Joseph Biden
Former Secretary of State and Retired General Colin Powell.
Though it would assure his victory, the Powell pick is too sexy for Obama. Sam Nunn has been out of politics a while, not unlike Dick Cheney, but I don't think Obama wants to draw any comparisons to Cheney. Therefore, I think he's going to pick Joe Biden.
The only other choice that makes sense to me is New Mexico Governor Bill Richardson. But I think Obama is counting on the Hispanic vote without him.
Historically, I'm not so good at predictions, so I wouldn't be surprised if he goes with the utterly unimaginative choices of Indiana Senator Evan Byah (yawn) or Governor Tim Kaine (who?) or even Kansas Governor Kathleen Sebelius (girl power). About the only thing that would surprise me is if he chose a certain blonde Senator from New York.