I'm a C9, at least. And generally I masturbate with the foreskin covering the head (no lube needed, you see.)

I have considerable foreskin problems with condoms. The forskin DOES tend to ride up and fold the rubber over on itself. You won't like hearing this, Ben, but cheaper, thicker ones stay in place better for me.

And roll them all the way on, down your shaft. The added surface area on which to grip seems to keep the condom in place more effectively, in my experience.

Enjoy!