Aviator, here's how I do it.....
Paint your figures with acrylic or other water-based paint first. Use care and paint like it's the final coat. Get all the details down, etc. Set aside for a couple days to make sure it's thoroughly dried.
After it's dried, mix a small blob of burnt umber oil paint (about the size of a burnt matchhead) and some mineral spirits. The oil pigments will mix with the mineral spirits and will take on the appearance of dirty thinner. With this thin mixture at hand, apply it to the entire figure, making sure you work the wash into the nooks and crannies. Blot away excessive wash.
Let the wash dry and go back over the high spots with a mineral spirit-dampened Q-tip to remove the wash from places it shouldn't be. The key here is to let the wash flow into the crevices and shadow areas of the figure and remove the wash from everywhere else.
Since your underlying paint is acrylic, no damage will be done by the wash and you'll be able to remove as much or as little as suits you. When done, let the wash set for a day or so before putting any finishing touches to the figure.
My first try with this was such a success that I actually am looking for ways to put figures on my models!
Good luck!
Dennis