Mark Wyner from Campaign Monitor wrote an informative article about testing HTML email in various mail readers and webmail providers.
We support sending HTML mail in MotorsportReg.com. I have seen some pretty good emails from a few of our customers, particularly ProCup Karting. Are you sending HTML email? If not, Campaign Monitor has published 30 tested templates that you can download and customize for your own emails. Using the "Source" button from the MotorsportReg.com message toolbar, paste the HTML from any of these templates and you can preview and edit it instantly.
With the great power of HTML comes great responsibility! Be careful about over-designing your messaging; remember your members are busy people but don't underestimate the value of good design in your communications.