Hi,
For a start my recommendation would be to take a look at the SketchUp-Artists page. There you can find plenty of descriptions on different techniques and styles.

http://www.sketchupartists.org/

For example have a closer look on how Oliver Shea does it:
http://www.sketchupartists.org/tutorials/the-quasi-clay-style-by-oliver-shea/

For sure you need to learn on how to do a line overlay:
http://supodiumforum.websitetoolbox.com/post?id=1578151

If you want soft lights and shadows (like in the second picture on the floor) you need to fake them in Photoshop or you have to employ a rendering software...

happy if this helps
alex