Analyze This: How To Sing Like Your Favorite Singer Without Injuring Yourself
The answer to the second part of the question is: the answer to the first part.
What I mean is this: People often have a mistaken assumption about what the singer (Christina Aguilera, Kelly Clarkson, Sting, whoever) is really doing with their voice. When they hear a singer reach for an impossibly high note which sounds strong, full and clear, they assume it's being done in chest or full voice, and when they try to sing the same note in their own full voice, they strain, squeeze, push and generally hurt themselves trying to achieve this high note.
What these singers have done, either naturally or with study and technique, is to sing these notes in their pharyngeal or 'mixed' register - in a way that you cannot hear a transition from the chest voice. They have learned to smoothly navigate between all their registers and place their sound where it resonates and rings. When you take your voice from the chest resonance, which is inside the mouth, and direct it to a place above your soft palate and forward towards the nose and teeth in an area commonly called the 'mask', it goes to the resonant place that those higher notes naturally belong, and you hear the 'ring' in the tone. It is clear and free and full, and it sounds as strong as what you are used to calling the 'full voice'.
Accessing that resonant spot consistently is what our exercises are all about. In preparation for singing this way, it is important to be able to identify this pharyngeal resonance in other singers.
You should develop a critical ear for this. It will help you in your own singing. When you listen to any singer, on the radio or CD or live or whatever, listen for the subtle differences in tone that will tell you whether a singer is in chest, pharyngeal or falsetto (head voice for women). Make it a habit to use this critical ear wherever you are, and you will be amazed at what you find. All your life, you assumed that people were taking their chest voice way up there, when it is simply not physiologically possible. It is possible, however, to develop and train the muscles that control your vocal folds, and manage your airstream so that you achieve this full sound in the higher register.
Remember that your ears are not a good judge of your own voice. What you hear inside the meat-box that is your head is very different than what other people hear. That's why people are always so shocked when they hear their recorded voice for the first time. Often when singers begin developing their pharyngeal voice, their complaint is always, "It sounds weak and wimpy - not strong like my chest voice." But this is a very serious misconception. Granted, when you are first learning this technique it may not be as strong. But if you listen to a recording of yourself singing in this register, you may be surprised. I just had a student who is a fabulous singer complain that when he tried to sound like a certain singer, his voice sounded false in comparison to the singer he was trying to sound like. I recorded him singing the same passage as the other singer and played it back to him. He realized that what sounded like falsetto inside his own head had a completely different sound outside his head, and his pharyngeal voice was as strong as the singer he was listening to.
So, the first step in developing your strong pharyngeal voice is developing a strong ear for what pharyngeal voice actually is. Choose a song, and then pick it apart - note by note! Listen carefully to the timbre of the voice; see if you can discern where they're making the switch. Listen also to the recording technique - if you listen carefully, you can tell where someone was punching in or out. Don't forget that when you're recording a CD, it's often recorded phrase by phrase - the singer may have recorded the high part separately from the low part. Also, think of all the takes that might have been recorded before they picked the best-sounding one to cut-and-paste into the track. And there's always pitch correction, and effects, and compression, and EQ, and a million studio tricks that can make even the worst singer sound decent, and a good singer sound impossibly amazing. So there are all sorts of things to consider when you compare yourself to a singer on the radio.
The good news is - you're a lot closer to singing the way you want to than you may think!