Always show markup toolbar in Preview
Solution 1:
In addition to the already mentioned Shortcut ⇧+⌘+A Preview in El Capitan (and Yosemite I guess) features a toolbar icon that allows to open the markup toolbar with a single click instead of clicking through the View menu:
To make the Toolbar visible, go to View > Show Toolbar in the menu. (Thanks to Rachel for this aditional hint!)
Solution 2:
No, the toolbar seems to reset every time a new image is opened.
However you can use the shortcut ⇧+⌘+A to toggle the option to show/hide it.