Maximize your warm-up routine. Begin singing at the lowest part of your range that is comfortable for you. Slowly work you way up the range to the point at which your voice begins to feel slightly strained. Work you way down again and repeat the process a few times to warm up your vocal chords.
Perform vocal exercises that switch between your low and high ranges. Concentrate on your breathing and using your diaphragm. Increasing your top end requires more breath and a firmer use of your diaphragm. Both can be increased with practice and by concentrating on how your body is responding as you push your range upward.
Focus on the way you are breathing. Your stomach should go out as you breath in. You should be able to feel the increase of vocal power as you breath in and by concentrating, you will eventually be able to harness that power to expand your range. Make sure that as you move your range upward, your jaw and neck are not becoming locked by the effort. Your jaw should be firm as you sing, but not locked in effort.
Work on your posture. As you sing in the upper register, your stance should be straight, but not rigid. Stand with your feet slightly apart, which centers your body and improves your overall stance.
Practice singing songs that are just out of your range. Sing without using a falsetto voice, and don't be distressed if in the beginning you are missing notes or your voice fails to respond the way you want. Don't strain your vocal chords, but slightly pushing your range as part of your regular practice routine will eventually expand your range upwards.