Hmm... lets see...
Some of these you may already know, but it actually took me a while to figure out (despite the fact that it tells you on loading screens xD)
Obviously, in easy, you're going to be playing with three fingers, when you get to normal, and past that, makes sure you use four.
If you have more than one note on the same fret (colour) coming up, you can just hold down that colour instead of pressing it each time.
If the song is slow or there's space between notes, then then press down the fret before you even get to the note so that you don't have to throw yourself off course right when you get there.
Definitely do the tutorial if you haven't already.
When you get to the point in the game where you do them practice, practice, practice your hammer-ons and pull-offs. Truth be told, I don't them correctly myself (though I can still do them). If you don't know what they are, they're in the tutorial.
Star power is a must sometimes, when your bar gets past halfway, and you see yourself about to lose, activate it and you should be good (as long as you're successfully hitting notes.
)
There's probably something I'm forgetting, but a lot of it is just practice. The more you play, the easier things will start getting. Don't push yourself too hard, as your arms will pay for it. At most, play an hour or two a day, and come back and do it the next day. Also, if you find yourself getting worse at some of the songs instead of better, that's your cue to stop playing and come back the next day (or any amount of time later). From my experience, when you come back, you're much better at it then you were before.
Then againm I haven't played for nearly a month, Perhaps I should be ready to go now? xD
(Freya was kicking my butt last time I played)