Alot of people who have toyed around with other 3d packages usually learn first thing how to make half a box and 'instance mirror' it so that you can see a whole symmetrical model. I've wanted it in sketchup for so long, but didn't realize how easy it really was (nor thought to actually google the solution until this post) until just now.
If you need visual aid, I can add it, but it's really simple.
Make half of a box (the size you want).
Make it a component.
Copy+move it right next to itself,
Then use the scale tool and hold shift to 'inverse' it.
You can now model symmetrically, and watch the changes happen as you make them. Someone might have covered this already, but I never found it