Obviously you need to have some level of coordination and rhythm to begin with.

I'd start off on Medium difficulty. Play your favorite songs until you master a few, get 5 stars on a few songs. Once you've gotten to this point, this is where the real training begins.

First, make sure you have a few hours to crack out on this...ALONE! You really don't want to be alternating with another person between songs at this point. (unless you both are willing to play the same songs after each other, and if so, then play 2 songs, then have the other person play the same 2 songs) Anyways, back to the tutorial. So you're finally on HARD difficulty... play each song IN ORDER, don't skip songs. Playing them in order will develop your skills for the next song and so forth. Keep playing until you can't beat a song or until you can reach the 5th group at least. Once you get to this point, you can now start on Expert Difficulty and follow the same instructions. Play all the songs in order until you fail... By this time, you should be familiar with the songs (which will help immensely) and I wouldn't recommend playing on hard again, it'll only make you worse. CONGRATS, you are now a guitar hero.

Additional tips: This is sort of obvious, but save your Rockout till the difficult parts of the song and/or the solo's. It'll help you pass the songs. Also, when you think the combinations are too tricky, don't think about it. Just trust your hands, you'll be amazed as they have a mind of their own.