Flat pieces and small pieces were the bane of my existence to prime or to airbrush. I tried alligator clips, keeping parts on sprue, tweezers...
In the end, I found an elegant and cheap solution. I staple masking tape sticky side up to cardboard. then you push the parts onto masking tape. Small parts, wobbly parts, flat parts that used to slide or flip away.... grrrrr... stay put.
It also makes it easy to get similar light sourcing across similar parts like space marine shoulder pads.
So other quick pictures below: