I created an "internal" test list and an "external" test list. When I'm tweaking the message I send to the internal list that includes all of my personal e-mail addresses and I have the "users" set for some to receive HTML and others text.
I just tweak as needed and resend to the internal list.
Then I send to the "external" test list for the business owners of the list and some friends who are copy editors to do a final review.
One thing I am doing is trying to ensure that I have customers from AOL, Hotmail, Yahoo!, Gmail, MSN, Cox, Comcast, Earthlink and other big ISPs on the list to ensure that my mail server is not being blocked at that particular time. AOL sucks.
Steve