Singing high notes can be a challenge for many. Often this is persons beginning to develop their singing voice. Also, it's a matter of life that after about 40 our vocal flexibility, just like the rest of our body, starts to not be as responsive. A further bane to hitting high notes can be psychological - a fear factor of hitting the wrong note or not sounding good. But, the positive news is that all of these situations can be overcome with a few simple techniques applied regularly.
Tip #1- Your starting point for all good vocal technique is good posture. Good posture is essential whether you are singing from a sitting or standing position. You need to be able to engage your diaphragm, fill your lungs, have control as you release air and so on. To achieve a good posture pull your body attentive like a soldier called to attention. Then exhale and whilst holding your firm position let your body relax. Now suck in a big, huge yawn and casually let all the air out - all the while keeping your good posture. Repeat the yawn a few time if you like to really open up your lungs and throat.
Tip #2 - Humming is a great way to loosen up your vocal chords. You don't need to generate a lot of volume so this can be done any a variety of places. You can use the humming technique in any place you feel comfortable like the shower or car. Make sure that your voice box is getting in on the action, that the humming is not just occurring at the lips. As you regularly work with the humming practice you can begin to expand your range, higher and lower, and work in different keys.
Tips 1 and 2 are good ways to warm up before a singing session. Go gentle with your warm-ups. You don't want to overwork your singing tools before you actually begin a session.
Tip #3 - Singing (or humming) the actual notes. You can practice singing your scales to a properly tuned piano, guitar or other instrument. Aim for accurate pitching and breath control. With regular practice you'll develop more sustain, volume, range and control.
Tip #4 - To hit a high note make sure you have enough breath in your lungs. Aim to be relaxed, lung full of air and mouth open wide.
Use the chart on this page as a reference. You'll notice it includes the 7 notes of the C-Major scale. Travelling through the notes of a scale in increments is an effective way to reach a high note, but requires more air and breath control. The octave jump is a technique as old as singing itself and still sounds great. Also, each scale has specific notes which make up the chord of that scale. They are called triad notes. Travelling through those notes sounds great. Remember to ascend and descend in all your practice sessions. Once you are comfortable with one scale octave begin to expand your range in both directions.
Use these techniques regularly and your voice will be in fine shape. The last tip is to relax and enjoy your singing - it makes for much better vocals.