When training, I try to keep heart rate around 150 bpm. I can sustain this HR for a long time. On Whitney, I've found that working in the 135 bpm range is better, especially once I hit Trail Camp. Hiking faster (ascending quickly, overexertion) can also bring on AMS.
For me it's a focus on my pace/breathing once I'm above 12K'. Not pressure-breathing necessarily, although I will do that from time to time, but keeping a pace that has me breathing deeply and steadily, but not in a gasping, oxygen-indebted way. I also keep an eye on consciously taking deep breaths when I'm resting at elevation.
