I love to see really thin girls (not annorexic or anything, in fact having a nice ass and tits would be a plus, just no chunky chicks) take cocks in the ass.
A good example can be found in Beata Undine's sets and in a lot of the TryTeens/GirlsGotCream/TeachMyAss/WhiteTeensBlackCocks/Etc girls.
http://peachyforum.com/forums/p/207938/740824.aspx
http://img221.imagevenue.com/img.php?image=73600_tryteens_123_397lo.jpg
http://www.white-teen.com/tma06/fhwl.html
http://www.white-teen.com/cream-rocki/fhwl.html
http://galleries.tryteens.com/pics/1212/aprilvcs/101113/
Take this "Nena" for another example: http://fhg.analteenangels.com/72011b/index.html?nats=NTUwOjU6ODM,0,0,0,48964
The rougher or more hardcore the sex, the bigger the cock, and the thinner the girl's waist and legs.the better.