If you're like me, and you LOVE to sing, you should check out the Ultimate Voice Training website. It's loaded with free vocal warm-up exercises. Since stumbling upon the site last week, I've already noticed an improvement in my technique. My favorite exercise is called the mum. It teaches you how to transition smoothly from low to high notes, and to keep your volume consistent.
Of course, nothing beats private lessons with a professional vocal coach. Back in the eighties, I had the honor of studying with Martha Bennett Oneppo, one of the best teachers in the world.