Tip A Day #12
When you plug in an external monitor, and you want to make it the primary monitor, it’s super simple!
Requirements:
Mac
Step 1
Open System Preferences. Click on “Displays”
(or just click on the monitor icon in your top dock. Choose “Open Displays Preferences”)
Step 2
Click on “Arrangement”
Step 3
Move the white bar at the top of one of the displays to your preferred monitor. Now your dock and files will be on that monitor instead.
That’s it!! See? Super simple! 
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good tip, is there a way to add a space/desktop and keep it present in a certain monitor? So i can have multiple desktops open in each monitor.
Great tip! One thing to look out for though – if the displays are positioned above and below each other in the preferences (rather than side by side) the menu will move, but the dock will always remain on the lowest display.
Yeah, you got it. But if you fiddle with those buttons and sliders in the Preference Pane too long YOU WILL GET CRAMPS IN YOUR HANDS! Ask me how I know … !
This solves one of my pet peeves every morning when booting my MBA to my old apple Cinema display via the dual DVI dongle.
Thx Cali.