I didn't go to school for makeup artistry, so everything I know is basically through trial and error. Here are some things I've learned about flash photography:
DO contour appropriately
DO use a foundation and powder a couple shades darker than your skin tone, or cover your neck and shoulders with your normal foundation
DO use matte finish foundations, concealers and powders. This includes blush. Eye makeup isn't as important to be matte, but it does make a more beautiful picture.
DON'T use shimmery finishes on your face. This will make you look greasy and gross.
DON'T use foundations or concealers with pink undertones. This will make you look pale or ashy.
Here is the difference between a do and a don't:
I used a matte finish foundation, concealer, powder and blush.
A light colored lip gloss with no shimmer and minimal glitter will accentuate you natural lip color.
On my eyes is just a black crayon that I blended upwards. I didn't use a highlighter.
When you contour correctly, there's no need to highlight.
I used my normal radiant finish foundation with a satin finish concealer.
I used a shimmery loose powder followed by a sheer coral blush and shimmery highlighter on my cheekbones.
You can see that I look really greasy and my face is lighter than my neck. This makeup is beautiful in person, but doesn't translate well in flash photography.
Here's what it looks like without flash: