My stylist gets amazing curl out of my hair with her diffuser, so I asked her to give me some remedial diffusing training.
One thing is that she swears by ionic dryers. I did end up getting one (not expensive) and it seems to me that it causes less frizz.
She also showed me that lifting the curls with the fingers of the diffuser works better than just aiming the air upwards into my hair, as I was doing.
I tend to have straightish top hair, like many 3as. She showed me how to use two medium size claw clips, clipped along the part to draw the two sides together, to give more curl and volume at the top.
I still am not as good at it as she is, but my technique has improved a lot!