Notice what you're doing. It's hard to do this because you may not even be aware of your bad singing habits. Pay close attention to your intonation and pitch.
Know that beginning a note under the desired pitch and then moving up to the correct pitch is a bad singing habit. It's called scooping.
Break the habit of scooping by repeatedly singing every note in the scale. Make sure to hit the pitches right on, instead of sliding up to the correct pitch.
Stop shallow breathing. Breathe deep from the bottom of your lungs. When you get good at it, you'll feel the breath all around your lower abdomen, instead of just the front.
Pay attention to how much noise you're making. A noisy breath is not a deep breath. Practice silent breathing until it becomes a habit.
Relax. Many singers are tense and don't even realize it. Forget about having a fat tummy look and breathe deep from your diaphragm. A good singing habit is to develop what's called a floating voice quality.