To put a frequently used file in the Finder Sidebar, just hold down the Command key as you drag the file and drop it 'shortcut' into the sidebar. You will see a line that shows you where you are about to drop the file's shortcut - let go when you see that line in the position that you need.
Don't worry - you are not moving the file to the sidebar. You are just moving a reference to it. It stays in its original location. Once you no longer need that quick access the file, just drag it out - you will see it disappear in a 'puff of smoke'! Very cute!
Here's a short video showing you how!