I agree with Batou that you don't necessarily have to start at the beginning, and write to the end. I know writers that write out all the scenes that they have ideas for, and then go back and string them all together, but I personally can't do that. I have to write from start to finish or I'll never go back and work on early stuff. xD
Anyway, what I do to get past writer's block is really to just keep writing, first off. Just write words down for it and see where it goes and what comes of it. Also, I listen to a lot of music. Then over time songs just start being recognized, to me, as being for that story. Then I start organizing playlists and making sort of 'soundtracks' for my story. So when I have no idea what to write, I just sit back, close my eyes, and listen to the songs and see what comes about from there.
Hope that helps!