I am a singer and think I've got a reasonable voice. I was struggling with the higher register stuff so took some lessons with a professional singer based in Leicester (can pass on details if anyone's interested). A lot of my issues were with mouth shape. The more rounded/oval I made my mouth and the higher I could raise the roof of my mouth the better the higher register notes became. You see opera singers do this all the time and it might look weird, particularly when you're singing a rock song, but it works.
Another thing we did in the lessons was sing a lot of different styles, from Frank Sinatra to Opera and that really helped. Plus I have lots of warm-up exercises, which I do mostly in the car before a gig to avoid looking like a complete silly billy in front of my band and the bar staff.