You should set a hot corner in system preferences for showing the desktop. Then you would just start dragging the file or folder, move the mouse to that corner of the screen and then drop it onto the desktop. you could also drag to the Finder icon and then use spring-loaded folders, or the Desktop icon in the sidebar of your Finder window. I suppose that a keyboard shortcut would work just as well as a hot corner, it just seems easier to use the corner if you're already dragging something.