Learn about the technical aspects of singing so that you understand the differences between the voices. The type of voice used by a singer is determined by the primary area of resonance. For example, a chest voice is one that primarily uses the chest area of the body acoustically, and a head voice uses more of the head areas such as the nasal passages and the hard and soft palate of the oral cavity. A singer's mixed voice is that range where the head and chest voice overlap and blend. The higher the pitch you sing, the more likely you are to transition from your chest voice, to mixed and then finally to your head voice.
Warm up your voice before you sing. A proper warm up includes breathing and relaxation exercises, as well as movement (such as tongue trilling and lip rolls) to the parts of your body involved in singing. Sing arpeggios using tongue trills or lip rolls, starting low and slow and working your way up through your vocal range.
Breathe from your abdomen. When you breathe properly for singing, your shoulders should remain still and your stomach should expand. If your shoulders are moving up and down as you breathe, then your breathing is too shallow. Imagine a string tied to your belly button pulling it away from your body as you inhale: if you visualize your abdomen expanding as you breathe in, this will help you take in enough air to sing with proper breath support.
Use your face, also known as your mask, for resonating your mixed voice. Drop your jaw and raise your soft palate. Keep your tongue relaxed and flat: think of it as a mat that covers the floor of your mouth, and extend it forward enough to touch your bottom front teeth and lip. Observe singers who are successful at singing high with vocal power and notice that their jaws are dropped, their tongues are visible, and the muscles in their faces are animated.
Practice often. Relaxation is critical, so have a place to practice where you are not afraid to make mistakes. During each practice session, spend some time at the top of your mixed vocal range and try to gently reach higher without slipping fully into your head voice.