We send out weekly bandwidth MRTG email reports. In addition we have setup scripts that will send out a alert if the customer should want to be notified if the server reaches a certain Mb/s level.
URL's for MRTG are also available, but we find dropping their MRTG graph with a explanation on how it works and what it means is very useful. Automation is the key I think in making sure bandwidth abuse is controlled.
http://reliablehosting.com/aboutmrtg.htm explanation on MRTG
We often time hear from new customers that they really don't understand the graphs.