Hire a voice coach. It will be very difficult for you to improve your voice on your own, as improving your voice means improving your technique in multiple areas. A voice coach will drill your intonation, your diaphragm and muscle control, your breathing and the ways in which you produce and connect your pitches.
Open your mouth wide when you sing. Of course, it is impossible to sing with your mouth closed, but the more you open your mouth, the more air gets into your mouth and ultimately into your lungs. This also gives you more ability to project your voice.
Open the roof of your mouth and the back of your throat. To do this, clench your tongue to the roof of your mouth; you will feel almost as if your throat is blocked. Remove your tongue and you will feel your throat and the roof of your mouth open.
Take full breaths. To do this, breathe in deeply and feel your stomach push out. This happens because your lungs are expanding and pushing your diaphragm down. The more air in your lungs, the longer you will be able to hold notes or expand vocal phrases.
Practice your intonation. To do this, you will need a keyboard or a piano. If you do not have a keyboard, you can get a small one at a music store. Play notes and then sing them back to improve your pitch.
Record yourself singing on a tape recorder or on a computer. Then play it back and listen to yourself. This will also help you to hear when your pitch is off.
Imagine your notes as rivers or waterfalls, or as some kind of gummy substance that bends or flows. This will help you to fluidly connect your notes so that your phrases don't sound stilted.
Trust yourself. When you trust that you know your music, that confidence will come through in your performance.