Relaxed Throat/Neck
Continuing on the theme of the importance of being relaxed while playing for good tone... relax your neck/throat muscles. While performing the tremolo for example, only the muscles that are involved in the opening and closing of your vocal folds are needed. While performing a bend, the tongue is the only muscle that needs to move. Engaging the neck muscles around your throat will narrow your air passage, diminishing the size of that resonant chamber, sacrificing tone production in that part of your mouth. Early fatigue is of course an issue as well when using more muscle than you need... so relax that neck/throat for good tone and a body that will last longer in practice and performance.