Since this is an LJ dedicated to crossover fanfiction, here are some essays about writing crossovers. But, remembeber, while articles and essays can be useful, they are not set in stone. So don't let yourself be handcuffed by what you read. Rules are made to be broken and all that. And, sometimes, my favorite parts of a fic are those that break the rules in a great way.
"A Brief Guide to the Writing of Crossovers." "Bridging Universes: How To Write Crossover Fiction" by Stephen Sobotka. "Crossovers" by Kadorienne. "The Ten Commandments of Crossovers" by Lucy Gillam. I actually disagree with a few of the "commandments." But some should definately be kept in mind.
"Thoughts About Crossovers In General" by Kathryn Anderson. Very useful essay. Especially the tips on devices you can use to bring fandoms together in your story.
Will be updated when I stumble upon more essays.