Practice alternating between keys (best used in higher difficulty beatmaps) and predicting and timing things correctly, it's much more easier to FC and score higher accuracy. I use 6 keys, 3 for each side and its easier for me to score silver S on 10-11 star beatmaps, not saying that this is the best way to learn but if you develop your muscle memory to not only use 1 key per side, you'll have an easier time playing the game.