The way I went about learning Lisp was to find an itch that I not only wanted to scratch, but could not conveniently scratch any other way. It's amazing how quickly you can pick a language up given the right motivation.

As I'm an Emacs user, this was quite easy to do. For those who are firmly wedded to some other editor, there are a number of other useful packages that can use various forms of Lisp for scripting; the GIMP, for example, uses Scheme.