Spend a few minutes loosening up your neck, back, abs and voice before beginning vocal exercises. Warming up properly is important to maintaining and improving your voice's flexibility and range.
Fill your lungs by expanding your stomach and lower back. Breathing deeply like this allows you to hold a larger amount of air in your lungs than you would otherwise.
Sing a few scales or simple vocalises. Focus on maintaining breath support throughout the notes, instead of cutting the stream of air short at the end of each word.
Imagine that each phrase is like a missile. It needs constant power to reach its destination, unlike an arrow or a bullet which explodes outward then loses energy as it flies.
Channel the energy in your expanded lungs to produce power in your notes. You can push with your abs to help when you're slightly underpowered, but if you fill your lungs up by inhaling, they'll have enough air to keep your voice afloat for long phrases.