Thank you all for the advice, it's really great hearing some opinions.

Here are my updated versions:

I changed the shading on his shirt and the color of his pants, and adjusted the color of his jacket and shoes slightly. I tried changing the color of his shirt too, but I couldn't find anything that really worked. In my original design there was a print on his shirt, but I wasn't sure whether that would be "too much"...
I think I'm getting somewhere with the designs of the two girls but his is a bit more difficult.

On her I tried adding the walkman, but I don't know how well that turned out. Other than that I made her pants a bit brighter (I think they look better now) and made her jewelry more visible.

I didn't really change anything on her except for the shading on her arm, but I added the print on her shirt, which I also removed from the original design (but probably shouldn't have).
(In case it helps somehow I'll put a link to the original designs down here, eventhough I already changed some things.)
http://i.imgur.com/lwBJr4n.jpg