And when they are big enough... make sure they can't tip over.

Snowy has toppled our over numerous times. It is close to 5 feet tall and heavy.
Once it punched a wall socket sized hole in the dry wall
Lucky I know enough about faux finishes to diguise it very well. Can't even tell it's there. One day he'll take out the TV. I know what you're thinking: Move it away from the TV. But, I swear there is not better place in the whole room. Unless it is dead center. Then we can't SEE the TV.

I was thinking of making a system of butresses for the base. Any other ideas?