[skishmonkey72]

Gun92 and emerald000 pretty much summed it up. I do have a thing or two to add though:

1) As for which songs to practice, I'd say practice two difficulties of songs more than others if you want to improve. The first is the songs you can combo sections of it, but you mess up in certain parts. Practice until you can combo larger sections of it, and eventually the whole thing. The second difficulty you should practice are the songs that are harder for you, so you can combo smaller sections (between 20 and 50 or so) but you lose your combo many times throughout the songs. Practice those songs until the patterns make sense and you can combo larger sections of the song.

2) It's fine to fail a song on purpose, it won't lower any of your ranks, as the game uses your highest scores for your average rank, and overall rank always goes up, never down (with a few exceptional cases, such as not hitting any arrows in a song). Just only do it when you're doing worse on it than you normally do. Never do it when you're doing better than you've done it before. Let how well you're doing determine whether you purposely fail a song, rather than how angry you get. If you do it every time you get angry, it'll become a habit and it'll make it about 50 times harder to improve your high scores. It took me about 4 months to kick that habit.

3) Play with your wrists raised. Raising your wrists lessens the bend in them, which reduces the risk of RSI (Repetitive Strain Injury) and CTS (Carpal Tunnel Syndrome). Playing with your wrists bent too much narrows the carpal tunnel, which compresses the medial nerve in your wrist, which is what causes CTS. The fact that you are moving your fingers accelerates the development of CTS with improperly placed wrists. I place importance on this because I once had CTS from playing 100, 000+ arrows per day while playing with my keyboard on my lap, bending my wrists 30 degrees while playing intense, fast songs one-handed. I couldn't play for about two weeks. So yeah, a tired, sore feeling is okay and normal, it means your muscles are tired, and they need to develop more endurance. But if they burn or hurt and you suddenly have a hard time moving your fingers fast, you may have injured your wrist and/or hand. It's a good thing you play spread, lessens the risk of CTS in most cases, your middle finger isn't thrashing like crazy as in one-handed play.

Wow, that was long-winded. Hopefully it was all helpful too.