The most important aspect of good singing technique is AIR. Singers must control their breathing or they become fatigued quickly and their singing suffers. These exercises will produce immediate results, and permanently improved performance if practiced on a daily basis. I have never yet seen a singer--regardless of the style--who didn't make instant improvement by doing these exercises.

Start by taking a deep breath, filling your lungs all the way down to the abdomen (not just the top half of your lungs). Then let it out very slowly in a constant stream. Imagine that you're exhaling through a very thin straw and the air is going out so slowly that you don't appear to be breathing at all--like playing "possum." It may help to picture a candle out in front of you, and your breath is moving so slowly that the flame doesn't flicker as you exhale.

Do this ten times.

Now do five more. But on these next five breaths, pick a nice comfortable note and hold it through the entire breath. Don't let it change in pitch or volume--make it seem like a key being held down on an organ. Be sure that each note is a comfortable pitch--somewhere in your normal speaking register. Low notes are good because they help the throat relax. Use a different pitch for each breath. Don't try to belt out high notes. That strains the vocal chords.

Now do five more of these, gradually CHANGING the volume for each note from zero up to a medium volume and back to zero over the entire duration of the breath. Always choose a different pitch for each breath and NEVER let the pitch go flat or sharp. In the case of these last five breaths all that is changing is the volume, and that should be changing at a rate that is undetectable.

For the last set of five breaths (this is now a total of 25 that you'll be doing) do everything you just did in the previous five, but change the timbre of your voice at a faster rate than the volume is changing. This is done by "sweeping" through the vowels: a,e,i,o,u. Make the change gradual in any order you wish. It'll sound like Tibetan chant, so if anyone asks tell them you've converted to Buddhism and you're atoning your spirit.

You may be asking yourself how this helps you and why results are immediate. The answer is remarkably simple. By concentrating on keeping your pitch constant you focus in on what your body needs to do to sing on pitch. Let's face it, pitch is the most common and important concern of any singer. Another side benefit of doing this exercise is that it pumps extra oxygen into your brain. The brain likes oxygen. In fact, the brain can't function without it. So more oxygen makes you think better and focus more clearly on the task you have at hand: SINGING.

When you're on the mic remember to keep the abdomen tight (imagine you're going to lift a piano) and the throat relaxed (like yawning), allowing plenty of air to move. If you need higher notes you get them with more air, not by squeezing off your neck muscles (a common mistake make by singers who burn out before they're ready to quit). Test this by imagining you see a friend across a busy street and you need to get his attention. Holler out "hey". You'll notice that you do this in a very relaxed way with lots of air. That's how to hit those notes that are right on the edge. RELAXED throat, pushing the air out with the abdomen.

One last thought: believe it or not, TALKING is more of a strain on your voice than SINGING (if you're doing it right). In fact, Broadway singers are under contract to not utter a single word on the day of the show until after the performance ends. So . . . if you want to be a successful singer you have to learn to breath, learn to focus, then shut up and sing!

References:
http://www.vocalist.org.uk/breathing_techniques.html