How to create a Smart Playlist in iTunes

Apple's iTunes music software offers three kinds of playlists: standard, Smart, and Genius. You can make standard playlists by just dragging and dropping songs from your library, and Genius playlists are made with a single click. Really, the only kind of iTunes playlist where brain cells come in handy are Smart Playlists.
So what exactly is a Smart Playlist? Basically, it's a set of instructions that creates a constantly evolving playlist of music.
iTunes comes preloaded with a few examples you can find in the playlist menu, such as Recently Added, My Top Rated, and Recently Played. You can tell them apart from other playlists because of their purple-ish color and the little cog wheel icon in the center.

To get a better understanding of how these work, let's look under the hood of iTunes' Recently Added playlist by selecting it, going to the File menu and choosing Edit Smart Playlist. You can also get this option by right-clicking on the playlist you want to edit.
Here we can see the rules this playlist uses. A Smart Playlist can include as many rules as you like, but this one only uses two rules: it looks at the "date added" information of your music collection and pulls any media added in the last 2 weeks; and it goes through and excludes anything that might be a podcast.
This playlist also has the Live Updating box checked, which means that the playlist is updated immediately any time content is added or deleted from iTunes.

To make something slightly more complicated, let's make our own Smart Playlist by going to the iTunes File menu and selecting New Smart Playlist. iTunes shows us a new window with the playlist instructions.
Now, let's say we want a playlist that will automatically collect all of our highest rated rock songs. There's a couple ways to go about this, so don't feel like you have to be exact. There's no harm in trial and error.
For this example, let's use the first instruction to tell the Smart Playlist to look for songs with the word "rock" in the genre description.

Next, hit the plus button to add another line of instruction where we'll say the rating needs to be higher than 3 stars.
Just to make things interesting, let's add another line where the skip count--the record of how many times you've skipped a song before it finishes--is less than 5 skips.

Now, you could go keep adding criteria, such as the date range of the recordings, or specifically exclude certain artists--but this should be plenty to get started. Under the instructions, let's check the limit box and set the playlist to limit at 100 songs--although you could also limit by a length of time or say memory size. By default, iTunes will choose songs at random, but since this is a playlist of highly-rated songs, let's have it select according to rating.
I'm also going the check the box for "match only checked items" since I don't want the playlist to include songs I've unchecked in my library. The "Live updating" will also get checked, so that newly rated songs will automatically be added or deleted from the playlist.

After hitting OK, iTunes will create the Smart Playlist and give you a chance to name it whatever you want. Remember, you can always go back later and tweak the settings by selecting the playlist and clicking the Edit Smart Playlist Option in the file menu.
That's it. That's how you create Smart Playlists in iTunes. You can use them to quickly generate playlists based around genre or decade, or even sort out the loser songs in your collection that always get skipped.