I'm playing Koussevitzky's Chanson Triste, however i'm having some left hand trouble, particularly in the sequence right before the cadenza. The problem I'm having is when it calls for positions with a half step in between the index and the middle finger, with a whole step between the middle and ring finger (kind of like spock if you catch my drift). I don't have much strength in my ring finger, and it always wants to slide back toward the other two fingers. Does anyone have any ideas to help with holding this position, or building the dexterity so that I can work toward it?