The most barebones version is very simple:
lein has a plugin called lein-cljsbuild. In your project.clj there is a way to specify the directories this program needs to watch for file changes. lein cljsbuild has an option "auto" which activates the directory watcher and automatically recompiles the cljs files when one of them changes.
This long-running program keeps the same jvm running for all the compiles, and lets you iterate more quickly.
How are you finding the ClojureScript REPL? I wanted to play around with it, but gave up after finding out how much trouble it is to get running. I was hoping it would be as simple as Clojure's `lein repl`.
It's a bit harder to set up but it's very much worth it once you have it running. I use clojure.browser.repl