How to indicate to visitors that they are using a test environment?

We often release a test version of our web service to a select group of customers. This version of the web service normally has a few bugs and anomalies (database being reset every day) and we would like to remind customers that they are using a test environment.

What is the best way to tell visitors that they are using a test environment?

• A banner across the top of every page with a contrasting background colour and relevant words, e.g. This is a test environment.
• A watermark across the website
• A different colour background across the website
• A "Alpha" or "Beta" type label added to the web services logo?

What is the best practice here?