Harmonica Angle
Sometimes students change the angle of their harmonica to perform a technique, feeling that the change helps them. Resist the urge to do this, your goal is to use the same angle of attack on the harmonica pretty much all the time.
What angle do I recommend?
If you allow the face of your harmonica to angle downwards slightly (around 30 degrees) you gain the benefit of using more of the top of your tongue, which is wider and more easily covers the three holes necessary for standard tongue blocking. This angle also allows you to open your mouth a bit more, placing the harmonica deeper in your mouth, helping to create a more relaxed embouchure and more bassy tone.
Though I feel this is the most efficient embouchure, there are plenty of players who play with a flat harmonica, so no need to change if you've already mastered the common tongue block techniques (slaps, pulls and octaves). What you don't want to do is play with the face of your harmonica tilted upwards, this commonly produces a thinner tone and makes tongue blocking techniques much more difficult to perform.