How can I perform a tactical withdrawal?

[Strategos Alexandros]

I've managed it with alt + click before but it could have been a fluke.

[overweightninja]

I've managed it with alt + click before but it could have been a fluke.

No, there is definately a keyboard shortcut to make your troops move while keeping facing, IIRC they still turn to move but put themselves straight back into formation. I believe its either alt + click like you said, or failing that shift + click, would probably say in the options menu.
Hope that helps.

[Puupertti Ruma]

According to a thread about almost (or exactly) the same thing as this, the alt+click is the way to have your units move while keeping face.