An obvious and often overlooked solution. That's what I did before the amazon lookup tools (which are severely restricted now).

I like dragging and dropping the artwork just because it's so cool to be able to do it.

Yeh i wondered about simply dragging and dropping to the 'Get Info' screen, but will the artwork actually save on your PC/Mac when you do this?

Yes, it saves it. It's how I do virtually all of my artwork.

You can also drag and drop any image file in the Finder or on a web page or other application, like FileMaker Pro, into the artwork pane (you may have to select "Show Artwork" from the Edit menu). You can also select a range of songs, get info, drag an image into the artwork field, then click Okay. The fastest way to clear artwork from a range of songs (e.g., when the artwork is incorrect), is to select the songs, get info, check the artwork checkbox, and without dragging an image into the artwork field, click Okay. This will clear any artwork stored, even multiple artwork.

The most elegant (and surprisingly inexpensive) solution I've found so far is iTunesCatalog. This application automates the process and gets good results. It can even transfer the images back to your iTunes library without you having to do so manually. I highly recommend this application to all Mac users.

Ever try Clutter? It's a small osx application will go out, gather the artwork off of amazon and put the album cover on your desktop or add it automatically to the itunes list. We all love it.
http://www.sprote.com/clutter/
It's open source, works great and it's free.

For windows users try http://www.ipodsoft.com/iArt.aspx

You might find this web app useful:

http://www.slothradio.com/covers/