The comspheres are integral to it, so it sleeping aboard the Celsius in every chapter. And telling Bahamut's Fayth that you want to see him again. And talking to moogle-suit Yuna while playing as Rikku during the very first chapter.

...I just used a walkthrough to get the perfect ending on my second playthrough. Easier that way - I can't image how long it would take to get it simply through trial and error.