A large part of the painterly look I get in my work is achieved BEFORE the image ever makes it to my computer.
It helps to have an extensive visual vocabulary and a knowledge of classic paintings and the techniques and lighting styles that artists used (and still use) so you know exactly what it is that you are trying to achieve (for instance - study the paintings of Caravaggio for lighting effects or look at how Rembrandt rendered skin tones and facial expressions).
Selecting the right model, wardrobe and set is also crucial. (i.e.- a glamour model with a fake tan and enhanced breasts is never going to look like she stepped out of a PreRaphaelite painting..sorry!!)
And perhaps most important is the lighting. Flat studio lighting is seldom conducive for painting. The play between light and shadow is something that most master paintings depict in minute detail.
Everything done in post should be merely an enhancement and rendering of what you captured at the time of principal photography (and all of this is coming from someone who is known primarily for "post-processing", but I always set things up in such a way that my post work will be seamless and organic...)