I've been playing with the demo version of a new application--well it's a plugin--for SketchUp. It's called LightUp. It creates the effect of multiple light sources and soft shadows with very little setup. Here's a screen grab. The demo version leaves a watermark on exported images and although I don't know why, it seems to square up corners on cushions. Still, it has a lot of promise for making more realistic looking images from your SketchUp models with a minimal amount of work. It won't be free but if you need the capability, the cost my be worth the ease of use.
And a sort of sketchy style of the same scene