The main ticket I usually imply is the music. That's what mainly got me hooked, and what sets my old obsessions and im@s is my favorite idols/characters have more to offer.
Plus watching your favorite idol(s) vids never get boring. :3
I bet also, if someone new to im@s gets favorite idol, then they'll come back for more. Since, the idol's job is to sell themselves and get people to love them!
This +1. I got hooked into iM@S solely by their musics.
If you want to spread the word about iM@S, the best thing you can do is to... well... go for the music route. That is, have them listen to the music.
The hardest fact for me is when my best friend said I'm into loli the first second he heard the voice in "THE iDOLM@STER" (pun intended)
I guess with the majority of the idols' age ranging from 12 to 17 at most (excluding Azusa, that is), they'll call it loli alright, as the "real world" idols people listen to are usually around 19-22++ years old.
Really, it's damn hard to convince them to listen to it when they've got the "loli" prejudice.
I did get him to listen to it 2 years later: after I complain about how I'm practically the only person in my hometown to listen to iM@S, he agreed to listen to them out of pity, and ended up liking arcadia and Cherry (mostly thanks to the high-bpm nature of 'em, just like K-ON!'s songs he listen to) but that's the only person to ever like iM@S that I know of.
I hope the anime will help lift iM@S's popularity, especially here (I hope up to K-ON!'s, that's more than enough for now). I'm fed up with having to import stuff directly from Hongkong or Japan when so many anime product stores are around and none of them stock iM@S related stuff in favor of BLEACH, Naruto, Fate, K-ON! and Yu-Gi-Oh!.