In addition to all the great points above, one other thing I continuously struggle with is the tendency to literally tense my shoulders when I try to play fast. For whatever reason, I began to realize that trying to play fast would subconsciously cause my shoulders to tense, which then caused my arms to tense, which then went to my fingers, making speed impossible. The only things that have helped is practicing the parts slowly, gradually speeding up, while paying attention to keeping my shoulders loose. I still catch myself doing it, but less and less.