Yeah, just practice. I've put several hours into it at this point and I'm still working on S-ranking songs in Star mixing on medium difficulty.
I can get S+ in 'Son of Sun' at that level, but 'Bamboo' still crushes me.
I'm pretty positive the game takes some sort of special pleasure in making my hands cramp up. I have the same problem I did with ddr, where I can keep the beat on faster songs and I get lost and tend to fail slower ones. Although with this game I am also struggling to figure out how to hold the fucking vita while playing. For now backtouch notes are turned off because I have enough to try and concentrate on. I'm enjoying the heck out of it but it is definitely a challenge.