Your iTunes playlists are not fixed in stone: you can add or remove songs, temporarily ignore songs, rename the playlist, etc. This tutorial focuses on "standard" playlists; iTunes' smart playlists are covered in previous tutorials. This tutorial applies to both smart playlists and standard playlists you created in iTunes.
Rename an iTunes playlist
You can at any time rename a playlist: this will not affect the content of the playlist, only its visible name. To rename a playlist in iTunes, double-click on the playlist's name: this will make its name editable. Now type the new name over the old one and you're done!
Open a playlist in its own iTunes window
iTunes lets you open a playlist in its own window, which allows you to work on the playlist while listening to something else in the main iTunes window, or check the status of downloads without having to leave the playlist, etc. To open a playlist in its own iTunes window, right-click (Windows) or control-click (Mac) on the playlist name, and choose Open.
Make iTunes Skip songs in a playlist
Instead of completely removing songs from a playlist, iTunes lets you temporarily (and indefinitely) skip them on playback. Simply uncheck the songs you got tired of, and iTunes will simply skip them until you tick the checkbox again. Note that un-checking if the song in a given playlist will not affect the status of the same song in other playlists, or in iTunes' main song list.
Remove songs from a playlist
To completely remove a song from a playlist, right-click (Windows) or control-click (Mac) on the song, and choose Delete. This will only remove a reference to the song from the playlist, not delete the actual song. Remember that items in a playlist only "point" to the real items, much like shortcuts in Windows or aliases on Mac.
It is also possible to edit the rules of existing smart playlists in iTunes: see this tutorial.