Hi
Ariana is very friendly for her fans

I sent her a request on 7/01/2011 and got it back on 8/03/2011
I included a lettre, my own pictures (3) and a SAE because I'm not able to get stamps from the US
She signed all of my own pictures and included a bigger one she provided herself, and a silly band, all in her own envelope (because mine probably was too small) and she paid for postage
But like ChipperJonesFan said, including a SASE and pictures increase your chances of getting a reply!
And whenever you can you should include a SASE (I do it whenever I can)
Good luck!