Can I make double-sided printing the default? In the "Print" sheet, there's a checkbox for double-sided printing, but it's off by default.  I often end up using twice as much paper as I want to because I forget to check this.

Is there a way to make this enabled by default?
Yes. In the Print dialog, check the double-sided box. 

Now, go to the Presets dropdown and click it. At the bottom of the dropdown list, you'll see "Save current settings as preset." Do that. It'll ask you to name the preset, so pick a name for it. 

The OS uses the last preset you used, unless you have checked the box to return to the default settings after printing. So in a sense, the last-used preset IS the default. But there's still a "Default" choice on the presets menu, and you can examine (but not change) its settings.