Hitting those high notes

Long time no post. I thought I'd try to get things moving again on the blog, but please, if anyone else wants to be an author on this blog, let me know. I'm sure there's many band members with interesting and valuable topics they would like to put forward and have discussed.

Anyway, I would like some help despite being able to (occasionally) hit the high notes (top D) in Donegal Bay, I am struggling big time. So, does anyone have any advice on specific practice/exercises I can do to help me get higher? Is it a mouthpiece related thing? I do a lot of lip building and flexibility exercises (I have John Ridgeon's 'How Brass Players Do It'), which help to a degree; will these help with high notes?

Any words of help and wisdom will be hugely appreciated.