I found that the easiest and one of the quickest ways to get a gold star on a song is to choose the following song and following power ups.
Song: "Song of the Century" by Green Day
Jackpot - Once you have enough overdrive, simply use this and hit all the notes, and you'll get a nice bonus.
Runaway Notes - Since they will only be on the vocal tracks, you won't have to worry about the tracks changing.
Super Vocals - For extra vocal points.
If you hit all notes without breaking your streak and use Jackpot during the "they're playing our song of the century" section of the song, I can guarantee that you'll gold star it.
First this song is only available if you exported it from Green Day: Rock Band.
Second, Not using power ups will only get you 4 stars. I tried it twice, first time without any to see if it was possible by merely FCing the song.
Hope this helps! :)
EDIT: Song of the Century is now purchasable on Xbox LIVE.