When I search online for breath control help I get approximately 1,823,482,396,482 results (joke) with tips on breathing in properly, but I cannot find even one about breathing out. I make sure to breathe deeply "with the diaphragm" as most people seem to recommend, but I still have a hard time making my breath last long enough without becoming tense. What are some tips for breathing out properly while you sing.
Read Richard Miller's The Structure of Singing: System and Art in Vocal Technique. If you are experiencing no problems in letting your diaphragm descend without tension and you are not over doing inhalation, then you are on the right track. However, be careful to take too deep of a breath, because this will rob you of the facility needed to exhale gently and strongly enough so that it does not become tense. Start with inhalation and assess that. You should feel the intercostal muscles surrounding your ribs move outward gently until you are ready to sing.
Next, you need to feel that gentle expansion of your mid-section in the intercostal muscles as you are exhaling. They should naturally not continue expanding, but there should be some opposition to prevent from collapsing and losing the breath as well as continual movement against the breath to prevent tension. This is called appoggio or "opposition". Feel that your breath is a continual line during exhalation with this leaning into and opposing your ribs', and diaphragm's natural tendency to collapse. This should provide you with enough gentle support and movement to not become tense.
When you get to the end of your air, take another gentle slow breath in long before you make your next entrance into singing, so that you do not create tension in these same areas upon inhalation. If it must be a swift breath, make sure that you do not have an audible breath. This means that your vocal folds and the epiglottis is in the way of the air entering your lungs free from any friction along the way to the lungs. What this does is create more tension in your larynx, which is the same problem of tension. Allow breaths whether fast or slow to be calm and completely inaudible, that means you have allowed the muscles that frequently are in the way (in many singers) to have relaxed in order to not antagonize your efforts. This relaxation will also help your tone. There is so much more to be said. Read the book.
When breathing out and having a note held for a duration of time, you want to bring the note out softly, and not to air out as much. You still have to be careful of sounding airy when doing this though. You want to project your voice while slowly releasing the air you breathed in. I find myself struggling with this too sometimes, with practice you’ll improve.