Cool! I always wanted to see a more active discussion of postworking renders. There are thousands of different techniques and tips. And even something as simple as loading a render into GImp or Photoshop and adjusting the contrast/levels or curves will improve many renders dramatically. :)

Alas, much of my postworking relies on a series of PS plugins. Everything they do can be accomplished by playing around with layer styles, channel mixing, color selection, and curves adjustment and so on -- which is what I used to do. But it's so much faster to use a plugin, and most probably wouldn't care to experiment for 20 hours on a single image, creating 100 different adjustment layers to test various effects.

Anyway, I will share this: The Photoshop mixer brush (right click brush icon, and it's the last in the dropdown list). With a 0% load (and the clean brush after load option on) and sample all layers checked, you can transfer samples from an underlying layer to a new one. When combined with different brush shapes and different levels of "wetness", this basically lets you "paint" over your render on a transparent layer, and automatically samples the colors from the render below. You can get all sorts of painterly effects this way, and the result does not look like a tacky computer generated painting filter because the brush strokes are created by a human -- you.

You can also use this technique with low levels of wetness and a simple round brush to fix small errors or to smooth out artifacts in skin tone, etc. And because you're painting on a separate layer, you can adjust the opacity to your needs or erase errant strokes without messing up your render. :)

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.