I have been in the same boat time and time again. I click one link, then another, then boom! Two hours of potential writing time gone. Here are a few practices I've taken on in an effort to curb that. Give 'em a try.

Scheduling
Organize your time. "I'm going to write from this time to this time, have an hour or so to unwind and grab a bite to eat, then resume writing at this time before calling it a day at this time." Writing it down helps.

If you need to do research for a specific subject you might be covering, set aside an hour or so before hand. If you're in the middle of writing when you discover you need to look up a subject, write down what you want to look for on a piece of scratch paper. Then, set it aside until after you've finished your allotted writing time.

Disconnect
From the internet, I mean. If you're hooked into a modem or a router, unhook yourself. If you're on a wireless connection, turn it off. If you can't go cold turkey, I think there might be parental control softwares you can use to limit your internet usage to a certain amount of hours per day. But don't quote me on that one.

Don't push yourself
If you really don't feel like writing, then you shouldn't force yourself. You'll only end up with a lackluster product. Just try to relax, finish what you're doing, and set up a time to get some proper writing done in the future.

I would also suggest taking a look at Scrivener. It's a writing program specifically designed for writing projects of all stripes - short stories, novels, scripts, research papers, work reports, and so on. The aspect of it most relevant to what I've been talking about is its full-screen editing mode. It's just like it sounds: the project fills up the screen and can only be dismissed through the escape key. You can't just click out of it.

One caveat: Scrivener was originally created for Mac computers. They have a PC version, but I've never used it so I don't know how it compares.

Food for thought, anyway!

Phobos is absolutely right. Paper and pen are invaluable. If you're having trouble focusing on the task at hand, cut yourself off from the source of the distraction. Ninety percent of my writing is done in a plain old composition book, usually while watching TV or relaxing after work; it keeps me from spending too much time in the depths of Cracked or sporking sites.

On the other hand, that's not to say that you can't enjoy the occasional bit of irrelevancy. And I wouldn't even call the Cassandra Claire mess irrelevant, to be honest--it's a fascinating study of human foibles and fandom screwups. The Ms. Scribe saga is my personal favorite.

I learned something about myself last year during NaNoWriMo. I cannot be trusted with an internet connection, if there is writing to be done. I spend more time looking at Failblog and Cracked than I do working on whatever I am working on.

So, how did I fix it? Simply put: notebook and pen.

I find that I get far more done if I am writing things out by hand, away from an internet connection. It is slow going and it is painful, at times. You have to deal with hand cramps and (in my case) sometimes not being able to read your own handwriting. In the end, though, it really worked for me.

-Phobos

Maybe it can work for you?

I too have recently discovered this and am reading a goodfic named Forward. It's AU, but explains that at the beginning.
Link for those who want seventy odd reasonable length chapters more stuff to read in their lives:
http://www.fanfiction.net/s/4099993/1/Forward

With my chronic laziness, I'm not sure that I'm the best person to answer this question, but I've found that getting organized really helps when I want to do work. I set myself a goal, like "I want to write a fic about [insert series you're currently fired up about]" or "I want to write a fic to practice writing action scenes". Then I start sketching out an outline and write myself a little plan so that I'm not just making it up as I go along. That way, I get to ration the excitement that comes with starting up a new project, and hopefully it goes on for longer.

Welcome back to the board. I've just come back from a stint of inactivity myself.

There's a whole internet show for that. It's called "What the F*** Is Wrong With You". It's NSFW, but the philosophy behind it is that laughter is the only weapon we have against stupidity. Come to think of it, that philosophy is pretty similar to what the PPC does...

Anyways, yeah. Just laugh. And believe me, I know all about the bad side of fandoms. One of my favorite fandoms is Kingdom Hearts, which is notorious for it's fanbrats squeeing over the bishonen characters (Which is pretty much every single original character in the series and the Final Fantasy ones), Mary Sue's abound, and ship wars.

So all you really can do is laugh.

You get tired of beating your head against a wall. Or I did, anyway. I still watch The Daily Show, but I gave up reading badfic, reading fandom_wank, etc. It just gets to be tiring, and there's enough to be angry over in this world without going out of your way.

That, and if I spend four hours on my laptop... if I've spent four hours writing and, say, building something in Minecraft, it feels good. If I've spent four hours reading badfic and going "AAARGH, why would anyone write that?!" it feels like I've wasted the day.

There's a lot of excellent 'fic in Firefly-verse. If you're into Sandman, search AO3 for "Endless Moments," it's a really awesome series crossover with Sandman and Firefly.