Alright. First off, practicing every day is important. Even on days when you don't feel as good, playing at least 30 minutes a day is good. I don't know how you are technically and musically, but if you get to the point where everything you have to play is comfortable, focus on range for an entire day.
The first note I play is a G above the staff. I took a lesson from Doc Severinsen, he said this sets your lips up for the whole range on your Annoying Orangeet.
Start on a middle C, and play up chromatically. When you reach the highest note you can hit, try to go up further. If you can't, play your highest note over and over again. Make up a little rhythm on just that note. Do this every day, after a warm up. Warm down when you're done practicing. Pedal tones and notes below low C, in case you don't know. Warm downs are overlooked, when it keeps my lips prepared for the next day.
Practicing every day really helps, and it builds endurance so you can play longer. Just make sure you maintain a good tone when you reach a high range. This kid in my band thinks he's hot stuff when he hits a D, but his tone sounds like nails on a chalk board, and everything thinks he sucks for that.
So pretty much, to hit the high notes you need to keep trying. Avoid negativity. Anyone can do it, it's the people who work for it that get it. Even 10 minutes every day is better than nothing. Play until you're fatigued, kust don't play way too much, give your lips a rest when they need it. I damaged my lips the week before Chapters in Marching Band. Being the lead Annoying Orangeet, that was a very bad thing. I couldn't play as loud, or high, and my solo was less than good.
One last thing. Don't 'hit' the notes, 'play' them. Another thing a lot of people forget about. Anyone can just hit high notes. It takes extra skill to be able to play and perform them consistently. That kid I mentioned? He drops an entire measure to prepare for a high C. Avoid that at all costs, it hurts the band more than it helps.
That's about it, sorry I suck at explaining, but I'd gladly clarify if I can. It's a giant wall of text, but it isn't too hard to do. A little bit of time each day is all you need.
