Apple - OSX (Yosemite) Make 2 sided printing the default for *all* applications

First save your print settings as a preset:

• Open a document, then choose File > Print. Click Show Details.
• Choose any print settings you want to save as a preset.
• Click the Presets pop-up menu, then choose Save Current Settings as Preset.
• Enter a name for the preset settings, then choose whether to use the preset for only the currently selected printer or all printers.

Then make that preset as default: hold the Option (alt) key when choosing your preset, and it will become Default.

Source: https://web.archive.org/web/20131005034126/http://support.apple.com/kb/PH10768

Thank you for that tip about the Option key. I spent some time solving this earlier today and here's another way: in a web browser, enter http://localhost:631/printers in the address box and press Return. After the list of printers is returned, click on the printer in question. It took me quite a bit of poking around to find the setting for single-sided/double-sided as a default (Print Type: under "Finishing Options" tab). I suspect one advantage to this technique is that it sets the default in the printer, where the Option key method may only set it for that print queue. Since deleting and re-creating a print queue is a standard troubleshooting step, the web browser technique might be a better long-term solution.