The current costume has come in for a lot of criticism. Including from me. There's just too much skin, and the amount of bared midriff has invited comparisons to Paris Hilton. Michael Turner's drawings of the character, in particular, exaggerated this by giving Supergirl an elongated, snake-like torso. I liked the original costume better, but I guess it's not hip'n'with it, or whatever the kids say these days.
Anyway, the "Draw Supergirl" meme is pretty interesting, because it's got a bunch of fun variants on the Supergirl design. The big issue is skin, and how much do you show. The skirt is also a problem: it's adorable, and it's a classic part of the outfit... but, y'know, she does *fly around.* While some folks have tried a design that's closer to Superman's, or even exactly Superman's outfit, I think Supergirl should show a small amount of skin -- nothing gross or sexualized, just enough to soften her a little, in a way that helps her come across as young. (Like Dick Grayson's old short pants did for him.) There are visibly powerful superheroines, but Supergirl shouldn't be one of them; when I think Supergirl, I think of a cheerful kid dressed for a warm spring day.
So: where do you go? Midriff, arms, legs, shoulders? I don't like the midriff, which is easy to over-sexualize, so keep that covered. Shoulders? A little too much for a kid. I love the skirt, because it's adorable, but at the same time it raises the whole upskirt thing. So maybe a loose skirt, a little above the knee, paired with some tights or leggings. Shoes: nothing fancy. Go with a variant on Superman's boots. I like the little white gloves some folks have her in -- that's actually a nice way to suggest a little daintiness and softness. For the same reason, I don't particularly care for the idea of Supergirl with shitkicker boots. To me, Supergirl is a girly girl -- you know, the sweet kid next door who can pick up a battleship. The contrast between her exterior and her massive levels of power is part of the character's fun.
I'd go somewhere different with the costume, but of all the drawings, this drawing and this one seem to capture the *spirit* of Supergirl the best, to me. Something in the eyes, and the smile. (The short hair on the first is really cute, too; I always liked Kara's old bob.)