A trick I used when doing an ACEO is I would put painters tape along each side of the back of the card so that roughly half was stuck to the back of the card and the other half would be left as overhang with the sticky side facing the front, then I would lay the card down on my work surface and use painters tape again to tape the over hanging tape to the work surface. This seemed to accomplish two things- 1, it held the card in place and kept it from moving around while I was trying to work on it, and 2, nothing got on the back. Oh, and I didn't have to worry about my brushwork going off the card, because the tape would catch it. Then, when I was done and the paint was dry, all I had to do was peel the card off the tape, and then peel the tape off of the work surface. Seemed to work for me, anyway.
. . .
If you go with the flow you'll either get washed out to sea or be dammed.