If you change your mind about a playlist's tune order, just drag the song titles up or down within the playlist window.
You can also drag more songs into a playlist or delete individual titles if you find your playlist needs pruning. (Click the song in the playlist window and then hit Delete or Backspace. When iTunes asks you to confirm your decision, click Yes.) Remember, deleting a song from a playlist doesn't delete it from your music library—it just removes the title from that particular playlist. (You can get rid of a song for good only by pressing Delete or Backspace when the Library's Music icon is selected.)
You can quickly add a song to an existing playlist right from the main iTunes window, no matter which view you happen to be using: Select the song, Ctrl+click (⌘-click) it, and then, in the pop-up menu, choose "Add to Playlist". Scroll to the playlist you want to use and then let go of the mouse button to add the track to that playlist.
If you want to see how many playlists contain a certain song, select the track, Ctrl+click (⌘-click) it, and choose "Show in Playlist" in the pop-up menu.