Specific training for mastering fast scale passages (runs) — breaking down into small intervals, slow acceleration.
Break the scale into small groups: 1-2-3, 2-3-4, 3-4-5 etc. Repeat each, accelerate. Join into whole. Use neutral syllable 'ah' or 'doo.'
Runs aren't a 'trick' — they're the result of muscle memory and precise practice. The brain must know every note before you sing it.
Mariah Carey practiced runs like a pianist practices scales — daily, slowly, with a metronome.