well there's different techniques to achieve different styles (some more harmful than the others), but the ones i posted above could apply as a general rule of thumb. proper breathing technique and getting strength from the diaphragm helps you hold notes more solidly and much longer. and they're great for belting out ballads! think Whitney Houston.
and i truly believe that anyone can become a good singer if just trained right and given the proper style for their vocal quality, timbre, and range. just like guitars and pianos, the voice is an instrument, and using it for singing is a skill that can be taught. it's just that seeing/experiencing people be "naturally gifted" with good vocal quality and strong relative pitch has become so much more common nowadays that it discourages others from even trying. life's unfair like that, sure, but it don't mean you can't ever reach those levels! you'll just need a little bit more time and training.