If the script is only minified for whitespace, some of the solutions offered may work. As you mention, JSBeautifier is a useful tool.
However, if the JavaScript you're looking at has been compressed with a tool such as YUI Compressor or Google Closure Compiler, the variables are going to be shortened and harder to understand.
To see this in action, compare the minified and unminified versions of jQuery (minified version uses Closure Compiler).
- original: https://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.js
- minified: https://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js
For the goal of learning JavaScript, I recommend learning from libraries and plugins written by some of the best JavaScript developers out there. Look at the source code for jQuery, YUI, Moo Tools. Look at code on GitHub by people like John Resig or Thomas Fuchs. Read DailyJs and look at his examples.
I hope some of these tools help you in the process of learning. These are the resources that have helped me most.