The other guides are very good, but I also found a challenge guide on xboxachievements.com that gives a strategy for each individual gig. These are very helpful for the specific challenges, and they provide a quick reference if you're looking for a specific type of challenge.http://www.xbox360achievements.org/forum/showthread.php?t=17...
As stated above, each song can give up to 9 stars, so you only need 808 of 909. That means if you can get 6 stars on a good number of songs, you won't need to Diamond as many challenges.
And, of course, the mic/fan trick works for all of the songs but 2, so if you're okay with fudging all the songs and wasting your day pressing the "A" button over and over again, you can get 99*6 = 594 stars without working.
You can also get Diamonds on "Steady, As She Goes", "Blue Orchid", and "Incinerate" just by turning on auto-kick/no-fail and letting it do the work for you. There's another 9 stars. Cheats definitely do not disable achievements... I got the feeling that by earning those rewards, the game is helping you along. (I just don't think they intended to help you THAT far along.)
So doing all of that, you should be able to get a total of 603 stars without playing a note. It ruins the fun of the game, I think, but there you go. Only 205 stars left to go. :-)