Practice breathing exercises. Many singers do not use proper air support, which does not allow your voice to fulfill its potential. When you inhale, your stomach should go out as the entirety of your lungs fills with air. If you find your chest is going out or rising, you are not breathing deeply enough. Practice breathing in and out. You can also take a small square of paper and hold it against a wall a few inches in front of you; breathe deeply and try to keep the paper on the wall by blowing onto it. If you breathe from your chest, you will only be able to keep the paper on the wall for a few seconds, or maybe not even at all. If you breathe from your stomach, you should be able to keep the paper on the wall for several seconds. While blowing onto the paper, sing one of your songs in your head. Ideally, you want one breath to last you for an entire phrase, about 8 measures or so.
Practice vocal exercises daily. Use pentascales and go up chromatically until you reach just past the top of your range. Pentascales are the first five notes of a major scale; you want to sing up and down the pentascale. Start at the lower part of your range and go up chromatically, meaning you are going to sing the next scale starting a half step up from the previous scale, and continue until you reach the top of your range. Also sing arpeggios, which are the root, third and fifth of a major chord; you will sing up from the root to the third to the fifth to the root an octave higher and back down to the fifth, third and lower root. Sing these exercises legato, or smoothly, and staccato, or short and detached. Do not try too hard or go too far, as you do not want to damage your voice.
Conduct your practice in front of a mirror. Check for proper breathing, posture and a relaxed body. You may also hire a private lesson teacher to help with these techniques.
Read about and practice proper vocal technique. The soft palette, or the top of the back of your throat, should be raised. To raise the soft palette, think about yawning and how open your throat is while doing so. Also try to reach the tip of your tongue to the top of the back of your throat. Overall, your throat should be relaxed; if you find yourself closing your throat to sing certain notes, you are not singing properly. If your throat feels like it does when you scream or growl, your throat is closed. If you are singing through your nose or if the pitch is flat (too low), you are likely not raising your soft palette and closing your throat. As you sing higher, you should open your mouth wider and use more air.
Follow a healthy diet. Avoid dairy products, especially before you sing. Drink water regularly; the water should not be too cold or hot. Avoid caffeine, even in tea. Do not do drugs. Drinking alcohol and smoking can also have a negative effect on your voice.