Good posture is essential for taking in the breath you need. Your back should be straight, and your shoulders relaxed. One good way to achieve this is to picture an imaginary string running through your body, with the end at the top of your head. Simply "pull" the string to straighten yourself out.
When playing a wind instrument you should take in air by tightening your stomach, not your chest like you normally would with day to day breathing. This stretches your diaphragm and enables you to take in the most air.
Air should be released in a smooth, consistent fashion when playing an instrument. How much or how little to release depends on the dynamic levels and volume the particular piece of music requires. Proper exhalation is one of the most important steps, given that it is the physical movement that actually produces the notes and sounds people hear.