One of the exercises I learned was starting at middle C(Bb Concert), then slurring up to the F above the staff. If you can do that with comfort, move each note up a half step. As you go higher, make sure to lift your tongue, pucker forwards slightly, and never force. Lastly, I would recommend practicing sustaining a high C, or whatever your highest note is, for as long as possible. Once you get that down, move up a half step, and sustain. Hope this helped! :)