There are many videos explaining the "Perma-Visage" method floating around on YouTube, but this one by Avatard shows the quickest, easiest method to "fix" the transmog options for Dracthyr looking to be a bit more fashionable.
Evokers have long been vocal about their limited transmog options. While in Dracthyr form, only very specific transmogs actually appear, but for those who want to play the true endgame of World of Warcraft, options are limited to specific instances that Visage form can normally be used in. While options such as Gleeful Glamour - Blood Elf exist, these carry the ever-limiting cooldown, and aren't truly your character - so for many, it's not the same.
Keep in mind, this is certainly an unintended bug, so use at your own risk. There's plenty to be said for adding this functionality as an actual feature for players, who have gone so far as to figure out ways to implement it when it clearly isn't intended, but for now, Blizzard doesn't look like they'll officially support perma-visage form.
