Well actually, the way the transformation into the Hulk is described is first required in order to understand how this is willfully done.
The process by which Banner usually transforms into the Hulk is presumably the result of the chemical catalyst, adrenaline (a.k.a. epinephrine). As in normal human beings, Banner's adrenal medulla secretes large amounts of adrenaline in time of fear, rage, or stress, which hormonally stimulates the heart rate, increases blood-sugar levels, and inhibits sensations of fatigue. Whereas the secretion heightens normal physical abilities in normal human beings, in Banner's case it triggers the complex chemical-extra-physical process that transforms him into the Hulk. The total time of transformation falls anywhere between seconds to five minutes, depending on the initial adrenaline surge, which will be determined by the original stimulus. Soon after the transformation, the amount of adrenalin in the Hulk will return to more normal, reduced levels. However, there have been instances in which Banner became the Hulk without any discernible increase in heart rate or adrenal levels, suggesting that the true trigger mechanism into the Hulk is far more psychological than physical"
In the 2008 Incredible Hulk movie, Banner is learning to control his breathing. Breathing and heart rate are connected. Your heart and lungs work in tandem. Presumably, if this effect is physical, he simply controls his breathing to increase his heart rate and quickly boost his adrenaline in order to transform.
If this is psychological, you have to look at the fact that the Hulk is an entity. The Hulk is a part of Banner's psyche which, according to the canon comic universe, is shattered due to a form of Dissociative Identity Disorder. Each part of Hulk is one of these identities. For example, the Savage Hulk, or the Hulk most people are familiar with represents Banner's anger. Banner himself is the emotionally repressed core state. Another example completely from the comics is "The Beast" which represented Banner's guilt, the mindless Hulk which was what Banner would fear the Hulk would become etc. etc.
So, presumably in his time meditating on the Hulk, he either became aware of the fact that Savage Hulk is an entity, and came to an agreement with him, thus being technically "always angry", or realized he can channel the Hulk's desire to simply be "left alone".