Breathe through the nose.
Make the o behind your lips, as in yawning with your mouth closed.
Don't push on the m with your breath, but let it hum gently.
Don't start the runs with a h.
Create your own one-syllable-words that start with a m.
Sing the exercise also on ng instead of the m while keeping your mouth slightly opened.
Anticipate on the vowel. Think of the oo before you sing 'moon'.