• http://lifehacker.com/5993307/easily-add-spacers-to-your-macs-dock

I really enjoy having more order in my dock.

An excerpt from the article quoted

Open Terminal and copy and paste this command:

defaults write com.apple.dock persistent-apps -array-add '{tile-data={}; tile-type="spacer-tile";}'

Then this one:

killall Dock

The dock will refresh with an added spacer can be dragged to any position (or removed by dragging it off the dock). Repeat the commands to add additional spacers.