Hmm...Well, when I'm sad, I just like for my boyfriend to lay down with me and hold me close to him. Cuddles are good. He's sweet by nature, so he tells me not to cry and that he's here for me no matter what. Helps me. :)
If you're not dating the girl you're wanting to comfort, just go do something fun with her. She'll most likely cheer up. :)
*Also can work if you are dating. >.>*
Hope I was of some use. :D