how do you prep your skin for foundation?

I always moisturize prior to foundation application. If my skin is being naughty, Ill add in a primer in between my moisturizer and foundation. This goes for liquid and powder foundations. Hope this helped.
 
Moisturizer and then the makeup.

If I'm going out to an even and if I'm using liquid, I try to use a primer, but it depends on my mood.

But I always moisturize before.
 
I put on moisturiser after I get out of the shower, then get dressed, then I will sometimes prep with YSL Top Secrets Flash Radiance otherwise I simply mix my foundation in with Clarins Beauty Flash Balm and apply that mixture. It works well for me.