To be successful in Dance Dance Revolution Mario Mix, I would suggest wearing sneakers while playing, and start the game in easy mode, practice and perfect all the levels you are having problems with, then move to a harder level.
Always keep both feet in the center of the mat. Choose what foot you will use for the up arrow and back arrow and continue to use those all the time. Most importantly, keep up with the beat of the music. If you lose the beat the game becomes very difficult.