Sorta goes without saying but like most things practice is what's going to help you out the most in the long run. However there are small things that might get you there quicker.
Make sure you have a comfortable grip on your Vita while playing, learn to play with one hand and multiple fingers since you'll need the other for the back touch which becomes more involved as you progress to higher difficulties, attempt to play with the song rather than simply reacting to the notes that do appear, and remember that Fever mode is an option that is highly dependant on your positioning - you don't need to pop it off as soon as it becomes available.
Some people also advocate playing on Hard difficulty from the very beginning to force yourself to better learn the songs rather than stumbling your way through to a passing grade.