There are many sources for lip slur exercises, but here (and here for trombone) is one that I came up with that I especially like (though I would not be suprised if this exercise has been written elsewhere). An important thing to keep in mind is that you play higher by bring the lips towards each other, and not by tightening them. I often see players who in tightening their lips actually pull them apart. At that point all you can do is rely on mouthpiece pressure, which is never what you want. Bring the lips together and have a constant wall of air pushing at the lips. The best way to keep a good airstream is to focus on your sound. Start down low with a good, resonant sound, and maintain that sound as you ascend. When practicing lip slurs it is especially important to use a metrome so that you don't just let the notes come out when they want to. You are in control.