For the past few weeks I've been working on a book, a sequel to the book I just finished about a month ago and I'm trying to get published, the sequel has progressed to about 40k words, I think I'm one-third finished but I've hit writers block and I don't know how to craft the rest of the story.
Meanwhile I have an idea for another story and I'd like to start writing it but I know I can't write two of them at once.
What should I do?

Either slog through and make a note to fix the crappy writing that will result or write a different scene further on, one you're more interested in.

You could also ask yourself if the scene you're stuck on needs to be in the story. A lot of people include boring scenes because they think they're important to the story and have trouble writing them because it's simply not interesting.

You could always start the new one and work until you get past that block. One of the writers I know has two series of books going at the same time. When she gets an idea, she writes until she's stuck or the story finishes all the while her mind works on the other series, as well. When she gets stuck on one, she writes down everything she's been able to come up with for the other story then goes back to work. Sometimes all you need is to take your focus off of it and let your mind work it out in the background.

My problem stems mostly from the fact that while I've already written the parts that set up the story and set it in motion and I have some notion of what the end will be like I don't know how to write the middle part and I'm getting frustrated with it. Inspiration for me works like water through an irrigation system and right now my inspiration is being diverted so to speak with much of my imagination now working on the new idea instead of my current work. Last time this happened I made up my mind to plot out the synopsis and the outline of my new idea then return to finish my old one. I didn't return and instead I spent the next three months writing my first actual complete work of literature.

i would do as GD sugested, just get off it for a while, let the water flow back on its own and when you get the idea, juggle back to the WIP.

Yeah, maybe. But as I said the last time I tried that I ended up getting immersed in the writing of the new idea and ended up finishing it. I have no intentions of going back to the story I left off. Actually the new idea I have is a sort of recycling of the characters and universe of the project I abandoned so in a sense I would be returning to it.

see, you fingered it out

I recycle a lot. Right now I have the ideas for three different seperate fictional universes: one's a fantasy universe (two attempts so far that have failed), a Superhero universe (one book that I'm trying to get published and the sequel which I'm now stuck on), and a Sci-fi universe (let me think here... 4 failed attempts, thinking about a fifth attempt)
Edit: Also does "Though the Stars Fall" sound like a good title?
Its a play on the Kantian maxim "Let justice be done, though the Heavens may Fall" which gives you an idea of one of the themes of the story.

Work from the ending and go backward. Write the ending first.

Well, that's what I'm gonna try, see how that goes. XD

But since you've got your idea already to 40K, maybe try writing scenes for the characters and see how those would fit in?

I've actually made my decision and have spent the last four hours writing the entire outline of my next work which promises to be really, really good.
Maybe I'll return one day and write the rest of the story. Who knows the first book might become incredibly popular and if so there will be demand for me to get off my ass and finish the sequel. We'll see how it all turns out.
Anyways thanks everyone for your advice.

You're welcome, Ducky

Although with your permission, I'd like to leave this open for other writers to ask for help with their own writer's block seeing as Nelly seems to be having it, too.

That's a great idea, go ahead.

My block stems from "wasting" times on forums like this. Which I don't consider a waste, naturally...but there's only so many free hours in my day (sometimes only free minutes), and I can't seem to force myself to sit down to the actual typing of the words onto paper (or into computer.)

I know it's a great story because I've laid it out to trusted listeners over the years, and the reaction is always overwhelmingly positive--which is encouraging--followed invariably by the question: "Why haven't you written it out yet?" to which I have no answer. TELLING it is easy. WRITING it is hard.

Bleah...

This is going to sound trite but the first step to writing is actually writing. You can't really call it writers block if you haven't written anything yet.
I don't mean to sound snarky. I suggest you open up a new document on Microsoft word right now and start typing.

.. Hey, Jonath? I've got a question.

How do I go about actually writing my story? I've got the plot, a memory from the two main characters' teen years (they're twins), all of the intricate details that only a hardcore fan will ever know (and that's likely to be just me who knows any of it), etc. But how do I start it? I know I said I'd start from the end, and I've already got the rough idea for the ending written down somewhere (need to check my notebooks again, I know I put it somewhere in one) but how do I go about writing the beginning? I never realized just how hard it was.

Well honestly the way I started writing was by setting aside a few hours and opening up a new word document. That doesn't help much does it? Well I really start out by writing an outline then writing the real thing.
Now once I attempted to start a book off in the middle of an action scene but you can also start it off with some plain old prose.
All the websites of writers will tell you the same thing: to be a writer you must first write. Infuriating isn't it?
Now the book I first started writing yesterday is a special case because I actually thought up the entire plot in the space of thirty minutes then wrote my thoughts down in two days. On the second day I wrote the first half of the first chapter. The easiest part of writing a book for me is writing the beginning, because a beginning can go smoothly even if you don't have an end in sight. What becomes difficult is slogging through the middle if you don't know the end or getting to the end if you don't know how to write the middle.

I think that's what's holding me back, getting to the middle; that's the part I dread.

I don't get the 'to be a writer you must write' thing. I do write. A lot. But none of it has to do with the thing I want to write. >_<

Thanks for the tips.

You could do what I did when I was stuck on developing my character for a book. Make an RP about it!

Heres some (well I think it is) helpful advice on starting out:
Think of it in terms of a movie. Your first chapter should open with a first 'shot'. Here's an example. This is the first page of the novel I'm working on right now:

“Build your walls high or don’t bother building at all”

-Ancient Thra’ha’ken Proverb

Chapter I

The Brown and Purple Figment

"The Losh tea had begun to take full effect on the beggar’s mind and body, he was drifting away from the mortal plane to the sublime home of dreams and illusions when his happy journey was interrupted by the sudden and rude arrival of a pair of hallucinations travelling down the empty, dusty street. He looked at the figments hard then he looked down into his empty bowl to see if there was any Losh left, to his dissatisfaction the Losh was all gone and when he looked up he saw to his frustration that the pair of illusions had not disappeared on the second glance as was customary for the other hallucinations he had seen before. They were a pair of persistent figments that were overstaying their welcome.

There was a brown figment and a purple figment; yes that’s what they were. They walked and talked like people but they couldn’t be people. They spoke in strange tongues and wore stranger clothing, their skin was pinkish-white and dark brown respectively and their faces were covered in hair. The beggar squinted hard and hoped they would go away and leave him to his happy daydreams; sadly they were quite keen on staying."

Now this character's point of view starts off the story, he's the first camera so to speak. But he's not a very important character. The pair of "figments" are important characters.
I find it best to start off like that, use something as a framing device for the story.

Here's an idea: You could base a tale of the story of Edward Mordrake. I would m'self but I'm far too lazy.

Here's an article on him: http://thehumanmarvels.com/?p=125

A song aboot him: http://www.youtube.com/watch?v=zGci5Tlur9o