It really depends on the badfic. Sometimes I like to read/watch the canon it was based on after I'm done reading it, but other times that makes the horror worse. In that case, I usually turn to other canons. Listening to Heart of Courage helps.

If I'm trying to get through a really terrible badfic, it sometimes helps to make an orderly, bullet-pointed charge list, organized by chapter if necessary. The illusion that I can do something about whatever travesties are being perpetrated by the fic helps out.

The first line of defense, of course, is my normal reaction to extreme pain: hysterical laughter.

My solution is to keep reading, but to read something else if the badfic is that bad. Badfics that aren't as scarring can provide a lot to laugh at. And sometimes I just retreat to good fanfics or read a real book.

I go read things from my good fic pile, or canon works that I find it easy to get lost in. The bad writing is replaced by the goodness.

First, I usually say "What" a lot. As in, five or six times in a row, pause, and then another series of them. Then I busy myself with a distraction. I'll go read a webcomic, or a book, or if it's really bad I'll find one of my cats for some quality petting time.

Seeing everyone act completely and utterly in character is like a soothing balm.

...That, or I put on the loudest, angriest song I have and bellow along with it until I'm too worn out to be angry. Then I go and read.

Nothing is better than Serious Sam or Ace Combat to vent off the anger and put the squicky things out of the head.

first place on the worst of the worst.

I break it down into small parts if it's long enough. I've managed to get through the first three chapters of C*l*br**n by reading one chapter at a time. Sometimes, I'm watching TV or videos at the same time, so that my attention is divided and can't get the full effect of the squick.

All else fails, I have a good brew in hand.

It takes a pretty high level to squick me these days, but if it really gets to me, I'll open a new text window and start writing myself. Everybody thinks "geez, I could write better than this," and I try to make the badfic a challenge to actually do it. Most of the time I don't produce anything worthwhile, but it sure helps. XD

I go to the social network sites I frequent and find something else to focus on. Failing that, I go read good stuff, like TVTropes or online stories. And I listen to music.

By which I mean, finding something in the bad fic to laugh at. Mentally riffing it as you go helps prevent harm from a lot of horrors, if you can summon the presence of mind (doesn't work on all fics, though, but it's worth a try).

Read good fanfiction. Like, legendary-status fanfiction. I actually read C*l*br**n the whole way through (seemed like a good idea at the time) and to cheer myself up I dove into an archive of awesome Good Omens fanfiction. Recently I broke my brain over Forbiden Fruit: The tempation of Edward Cullen, and revived after reading most of the contents on the Sherlock Fanfic Recommendations page on TV Tropes.

Or immerse myself in canon! I read Tamora Pierce or Redwall, or I watch Sherlock and NCIS and Firefly and it makes me happy.