I'm not sure what your level of experience is or whether you have had instruction, but there are some basic things you should be doing anytime that you sing, and especially when singing notes at or near the top of your range:
Smile the entire time. Or don't smile, grimace. Either way, raise your upper lip towards your nose in a bit of a sneer and take some breaths, so you learn the way that it feels when your soft palate is raised.
Breathe as though your mouth were at the small of your back. This is easier if you are sitting, and almost unavoidable if you are lying on your back. You're still smiling, right? :)
Once you are doing both of these things, make sound, and use your diaphragm to support as well as push the sound up and out of you. Try to imagine that the sound is coming out of your nose instead of your mouth.
Try not to think too much or at all about the fact that you want to sing a high note. These thoughts will shake your confidence, lower your facial muscles and thus your soft palate, and choke your sound.
I really hope this helps you at least a little bit.
EDIT: I wanted to suggest that in trying to hit these notes that you work first with wordlessness, such as the backing vocals in "Little GTO" by Ronnie and the Daytonas. It is far easier to concentrate on hitting difficult notes if you aren't also worrying too much about forming syllables.