You'd stretch your legs if you were going for a run, otherwise you could sustain an injury. You should treat your voice in the same way you'd treat any other set of muscles. The larynx, jaw, tongue and diaphragm are all prone to injury if used improperly. Do breathing exercises, yawns, mouth-stretches and scale warm-ups before every rehearsal and vocal performance. You can purchase warm-up tapes with piano scales that you can sing along to. The benefit of these is that each scale is named so you know what you're singing.
If you're trying to sing songs that are too high or too low, this could be one reason why your confidence is not through the floor. Every singer has a range, outside of which the voice isn't as strong. A good singer knows her range, she knows when to stay inside it and she knows when to take risks and move out of the comfort zone. During warm-ups, make a note of the major scales that you can sing with the most ease.
The way you stand directly influences the amount of projection you can achieve. Projection is power from the diaphragm that adds volume to your singing. If your diaphragm is compressed by a hunched singing position, you will never optimize your projection potential. Stand with your feet shoulder-width apart, relax your shoulders and push your chest out. Inhale through the nose and use the diaphragm to push out when singing.
Use your cell phone to record yourself when practicing. If you have a "Voice Memo" function you can let it record as you practice. If you have a video-camera function, set up your cell phone so it captures the sound and visual. This way, you can review your performance at your leisure and make a list of notes about where you can improve. Watching footage back can answer some questions about your singing technique. For example, you may not notice that your posture changes throughout a song. This could be the reason why you can't nail that tricky crescendo.