As Mendez said, you must learn to play slowly before you learn to play fast. I was taught to "bang" your valves down when playing slowly. By "bang" I mean push them down as if you WERE playing quickly. This technique prevents lethargic valve pressing when playing slow passages. Lethargic valve pressing can and does result in clams, missed notes entirely, and a lack of smoothness in soft, slow playing.
Many good recommendations here and strive to make a fingering weakness a strength. Learn the sound of each valve combination and even very tough, finger twisting passages will come out clean.