To be completely transparent, I got this recipe from my best friend, Emily. After trying it along with several other recipes I found I liked the consistency of hers the best, at least with the kind of paint I typically mix with it.
1/2 c plaster of paris
1/2 c water
2 cups flat paint (I typically use Behr Premium Plus from Home Depot)
Mix the water and plaster of paris together until its very smooth. Slowly add in the paint, 1 c at a time, stirring. Keep sealed when not in use.