I found this tutorial, especially the sections on domain mapping, to be extremely helpful for doing things that I was banging my head against the wall to do.
Today we will be discussing a few very important aspects of WordPress Multisite. If you are new to WordPress Multisite please go through the WordPress installation tutorial first to get an overall idea of the Multisite feature in WordPress. In this tutorial I shall be discussing a few key things essential for WordPress Multisite installation, along with some common troubleshooting tips. Finally I shall discuss WordPress Multisite Domain Mapping in detail.