Some things I do:

Diffusing and/or bounce the flash (even if it is on camera). Use a tissue or be more creative.

Underexpose a little with the whites, as it is easier to bring out detail from shadows than to bring back blown highlights.

Here is a white kiddie i did with a canon 20D and 85/1.8 (and I postprocess). From the looks of the catch lights, I think I was using a mini soft box on a sigma 500 dg flash mounted on the camera.