Here is some stuff about documenting your code. I know this is something that you do regularly, and always make sure to keep up-to-date. Not.
The Java SDK comes with a tool that is easy to use (really) called Javadoc. It does an amazing amount of work for you by taking the API documentation for your applications off of your shoulders. This out-of-the-box functionality lets you write special comments directly in your code, and then it generates formatted HTML files to document your application's API.