Probably one of the things we brass players have the most difficult time with is our high register. There are several things we should remember when working on those high notes:

1. The most time we spent up there, the better it'll get and the easier it'll become. Just like any other aspect of our playing....if we want to improve we need to spent a good deal of time practicing it.

2. Blowing more air is not necessarily the best way of approaching it. Remember that air quantity and air pressure should be inversely proportional. In simple words: the high register needs less air but blown with more pressure. The low register, on the other hand, needs more air with less pressure.

3. Stop tonguing so hard. Try playing Bolero without tonguing at all and record yourself. You'll notice how it sounds articulated enough. Remember that due to the nature of the trombone partials are closer in the high register.

4. High resgister is like planting a seed. It'll be small at first but the more we work on it the bigger it'll get.

I hope this helps a little bit.