How can I add Trash icon to the sidebar in Mac OS X Lion?
First, open the trash can by clicking on it in the Dock.
In the Finder, press Command-shift-G (go to folder) and go to ~/.Trash
In the Finder window for .Trash, click the proxy icon in the title bar of the window and drag it to the sidebar.
You then have a trash folder in your sidebar.

To empty the trash from the sidebar, open Automator and create an application.
As the sole action in the application, select "Run AppleScript" and use this AppleScript:

on run {}
  tell application "Finder"
    empty the trash
  end tell
end run


Save the application as "Empty Trash", and drag it to the sidebar.