Rightclick - exclude from snapping

[mitcorb]

Make the imported graphics a group or component, if it is not one already. It is best to trace over the imported file. This is the preferred and recommended method.
Make sure that you import a clean, simplified copy of the file you have received, so that the original can exist in all its glory as it was received.

see this vid: http://support.google.com/sketchup/bin/answer.py?hl=en&answer=86648

[Dave R]

Make an image file from the CAD file you get and import that as an image. You can either scale that and trace over it or do as I often do and put the image in the background so I can look at it for reference while I'm drawing.

Here's an example of a table I drew a few years ago. The drawing wasn't anywhere close to being usable for tracing but by standing it up in the background it gave me an easy references for drawing the table anyway.