Static files can be served with express' static middleware. I usually make a directory for all static files to be served from the root.
If you've installed express globally with npm (npm install -g express) you can type the following at the command line.
express <project_name>
This will create a small example project for you. This example project has a folder named public, from which it serves static files. It further contains folders named javascripts and stylesheets.
The relevant piece of the example project for setting this up is the following line in the file app.js in the function passed to app.configure.
app.use(express.static(path.join(__dirname, 'public')));
Example is from express 3.0.0rc1
Express is built on Connect. The docs for it's static middleware might be helpful: Connect : Static