How to set duplex printing as default

In order to save paper, PLEASE print in both sides (if printer supports it). To set it as default, follow this recipe:

Windows

1. Go to Printers (& Faxes) Definitions
2. Select printer
3. Printer preferences (left tab or right mouse button menu)
4. Select Duplex printing

Mac OS X

1. Select Two-sided in the Layout options
2. "Save as" new preset in the Presets options (e.g., name it "Duplex")

The selected preset is persistent across applications and sessions.

Linux

Gnome desktop

contributed by Giampiero Salvi

1. go to System -> Administration -> Printing
2. select the printer
3. click on the "Printer Options" tag
4. choose "Long Edge (Standard)" in the "Double-Sided Printing" row.

KDE desktop

contributed by Hugo Costelha

1. run "printers"
2. right-click "Configure" on the desired printer
3. where it says "Duplex", choose "Long Edge"