That's simple. You tax sitting on money not earmarked for a recognized task (EG: Saving up for big investment X (named) such as college, a new home/building, etc.) or for rainy day funds; but you also limit the maximum that can be stashed for such tasks.

Then with that tax you re-invest in infrastructure, exploration of new resources, and even missions to space. (I'd rather we have a (robotic) mission to the asteroid belt than mars...).