It seems like Warner and DC are working to get Justice League out by 2015. Most belive that they should do what Marvel did with The Avengers and have a build up to it. With that said, this is how I think the build up should go, if Warner and DC go that route. Knowing them, they won't, but we'll see. I'll put them in the order that makes the most sense to me. I'll also include how to tie the films together.
1. World's Finest: This of course needs to be first. Without a doubt, Batman and Superman are DC's most popular characters. A film with the two of them in it makes the most sense. That way, you can set up not only them, but Lex and Joker as possible villains.
2. Wonder Woman: She is DC's third most popular character, so naturally this would be next. Of course, this would be an origin story. But a good way to set her up to be in the Justice League film is to have her end up in either Gotham or Metropolis.
3. The Flash: While not in the popularity level that the first three are, Flash is a key member to the team. No suprise it'll be an origin story. I think the best way to tie him up to the JL is a cameo by the Martian Manhunter. That way, like with World's Finest, you get two heroes for the price of one.
4. Green Lantern 2: With the first film done and over with, some debate on whether a reboot or sequel should be made. A sequel would continue the story, and a reboot would start a new one with John Stewart. I think a sequel would be the best way to go. Tie him in with another cameo by the Martian Manhunter at one point in the film. You can also set up Sinestro for a possible villain.
5. Aquaman: This would be the last of the lead ups. The last origin story, with a similar ending to Wonder Woman. Aquaman ending up in Gotham or Metropolis. I personally think that he should end up in Gotham City and Wonder Woman ending up in Metropolis.
6. Justice Leauge: This is where it all comes together. The seven heroes coming together to fight against not one, but 3 villains who have also formed an allaince. The League will be up against Lex, Joker, and Sinestro. At the climax, Darkseid arrives and the battle gets worse for the team and the world.
Well, that's how I think DC and Warner should do a JL lead up. I don't think they'll do this, but one can hope. Let me know your thoughts, and how you think it should go down.