Use exercise to increase your stamina and endurance while singing. While you exercise, your red blood cells don't get as much oxygen into your lungs. It mirrors the effect of being under pressure while on stage.
Choose aerobic exercises. They help develop your lung and heart strength, along with your endurance. Focus on your repertoire while you're walking or running on the treadmill or playing tennis.
Know that you're increasing your lung capacity with aerobic exercises. Thus, your voice projection will also improve.
Do yoga or Tai Chi to improve your relaxation and body awareness. You will learn relaxation techniques that you can use on stage.
Practice, practice, practice. The more time you spend singing, the more strength and stamina you'll have.