i usually ask first if they know them!!! i play the most known songs to see if they do or not :D
then depending on the vibe i just pick what i think they would like best...like this girl on my english class last year wanted to get into mcr because i gave a presentation about the demolition lovers and she was into like love songs and really liked demolition lovers so i recommended almost all the love songs mcr has to her :)
I've recommended to my friends to listen to anything from the revenge album or bullets, specifically Helena, I'm not okay, Drowning Lessons and Our Lady of Sorrows. (I also tell non-fans to listen to destroya :))