Mental Ray renders are great, but sometimes you want something softer, more impressionistic. There are many ways of achieving this, but ideally I want to stay as close to the Revit workflow as I can. So I aim to limit any image manipulation outside of Revit to what I can achieve in 10-15 minutes.
You can see some examples on my "Revit work at GAJ" page
I usually start be combining a shaded or hidden line image with a render. Make sure they are both the same resolution, then it's just a simple ctrl-A, ctrl-C, ctrl-V to get one layered on top of the other.
Next step is to duplicate the background, apply a watercolour filter and fade this new layer to say 40% transparency
Finally, duplicate the hidden-line layer, and set the mode to overlay. Add a layer mask, and make the centre of the mask partly transparent by applying black with a big soft brush set to say 10% opacity. You are looking to create a mask that fades from dark grey in the middle to white at the edges, basically a big soft oval vignette. This creates a subtle fading towards the edges.
With practice you can create very nice images in less than 10 minutes. For best results you need to fine tune the transparencies and final image contrast levels to suit that particular image.
Some more examples below, including a "squiggled" DWG export overlaid on a live Revit view.
Combining rendered views with hand sketching can be very effective. This one was done to communicate our comments to a specialist rendering service employed by the client.
Have also experimented with Autodesk Impression. It's a free download, but you have to do a DWG export first, so the workflow gets a bit too elaborate for regular use.
One last thought. Wouldn't it be great if this kind of image could be produced internally in Revit. Some kind of plug-in maybe that allows you to combine a hidden line view with a render and set the transparency. I suspect it's beyond the capabilities of the API at the moment, but it would be very nice if you didn't have to leave Revit, create a jpeg, delete the old one from the sheet then position the new one carefully in the same place.
see this post Ronchy Images for more on image processing.