How to do a html Mailing list ?

[eluser]Clooner[/eluser]
I made a program inside my app to deliver html mails to my subscribers. This works just perfectly fine for mail programs like apple mail or outlook but if you use mobileme, gmail or any other web based app the messages get messed up. Are there any tricks, guidelines to make sure the mail is also correctly formatted in web based mail programs. I know about the alternative text message which is included in the mail library but I'm really looking for tips and tricks on how to improve the html side of my message.