Smelling a Rose - "Backwards"
I know it sounds crazy, but I believe that is how your throat position should be for playing trumpet. Notice that when you really try to smell something how open your throat is? Well, remember that throat position, and play your trumpet. Yawning is also something to think, however when you think about yawning you usually do it. (Yawn!) There's a device called "Air Enhancer" by Alan Colin that's a great idea. If you don't have one, you can accomplish pretty much the same effect by trying to put all your fingers in your mouth and breath in. Then remember how your throat position was. "Your throat has a memory"
I disagree with teachers who teach the use of syllables. That brings the vocal chords into play which restricts air velocity. Also there are differences in player's throat sizes. I know I can swallow a whole handful of vitamins without water. I don't recommend that, it's much easier with water.