Let's start with the first thing your reader will see.

The Title.

I don't know about you, but if something's in a different color and a few font sizes bigger than everything else, then that's what I'm going to read first. Come on. How many of you went straight to the blue before noticing the irrelevant sentence above it?

For both the title and the summary, follow general conventions. The rules are a little looser for crack, but you generally still want to sound halfway intelligent. Just because it’s crack, it doesn’t mean “***:3 MEET DA SHINEEEE GANG!!!!!111 :D*** <3” is gonna fly. Just don't spaz too much.

You can probably pick out crack fics by their titles. There’s little chance “Little Pink Bunny Hat” and “The Excellent Adventures of Princes Kyuhyun and Eunhyuk” will be serious fics, and you can attract people who are looking for a laugh this way. But, well, you could give your crack fic a heavy, pretentious title if you wanted. No one's stopping you. 

Again for the summary, I recommend making it really really obvious that this is a crack fic right up front. Humans have this lazy syndrome where they don't like clicking on a link until they're absolutely sure it's something they're willing to spare at least ten seconds on, so you want to sell your story as much as possible when everything's in plain view. No extra clicking required. Get right to the point too, since AFF does that thing where it won't show the whole summary.

Example:

"You know it's a good day when your palace court is sabotaged by an insane lady who may or may not have rabies, and the ramblings of said lady convince your parents to send you on a quest for true..."

Even though you can't see the whole summary, you know right away that it's crack fic. Mostly because of "rabies."

OH, AND WHILE WE'RE ON THAT TOPIC.

AFF uses "Description" instead of "Summary", which I believe is what every other fiction site uses. Then it says "Foreword". The thing people are going to see is "Description", not "Foreword". You probably want to ignore the actual definition of those two words and treat "Description" as "Summary" and "Foreword" as either "Oh by the way" or "Here's an excerpt".

Avoid making it too wordy and/or giving away all of your jokes before anyone starts reading.

All in all, a crack summary isn't all too different from a normal summary. So, moving on!