Here is another fine gallery of exceptionally beautiful girls wearing yukata, kimono, and various tasteful accoutrements of traditional Japanese attire, although some of the girls do seem to be distinctly not wearing their yukata.
As a commenter in the Vipper thread from which I pilfered these images said:
Girls in yukata must have small breasts
They must have their hair in such a way that the unaji, the nape of the neck, is visible
Makeup must be applied only in moderation
If they are holding a pouch with both hands: more moe
Undeniable wisdom in those words, I think? I think it also goes without saying that they must have black hair free of henna, though I think it might be asking a little much for traditional Japanese ladies underwear (none), unfortunately.
When you’re done, may I draw to your attention the earlier yukata gallery (chiefly 2D), the comprehensive miko galleries (again 2D), and this recent 3D gallery courtesy of Danny. You can also see these images and more, and contribute your own if you are of a mind, on Sankaku Channel.