If you recently upgraded to Linux Mint 16, you may have noticed that your Places menu no longer has links to the Videos, Documents, Downloads etc. folders.
To customize, simply create a file called .gtk-bookmarks in your home directory, with text such as the following:
You can create a shortcut to any destination.
Tested on Mint 16
EDIT March 2, 2016: Using Ubuntu 14.04 / Linux Mint 17+? Create a file called bookmarks in ~/.config/gtk-3.0 using the format noted above. If the directory doesn't exist, you can create it.