I know what you mean! I have no idea how those girls do it. Most people tell me I'm pretty and I guess I have a nice face, but I can't help but look like a mess all of the time - my hair always frizzes out, my foundation never stays nice and my skins ends up looking dull or patchy etc. Whenever I go to gigs, I always end up looking terrible because of all the moshing/dancing etc. yet so many girls look absolutely fine (and yes, they've been dancing a lot too.)
I think it's partly luck (eg. those with naturally clear skin and glossy hair are always going to look more 'perfect'), partly spending a lot of time primping and finding the right products, and partly putting outfits together well, eg. I think a lot of these girls look more polished because they've taken the time to put outfits together and complimented them with polished nails/accessories.
Also, I think a lot of the time these girls just look confident and comfortable, rather than readjusting their clothes all the time/slouching rather than ~walking tall. The way you carry yourself can make you appear more polished.