As someone who learned HTML and CSS in the 2000s, it's really hard for me to give up my CSS and code in tables. But the more I design and create email templates, the more I realize that I need to get over this complex. Here are some more best practices that I've learned about designing, coding, and sending emails.
Keep It Simple
I've designed really complex email templates before, usually at the request of a client. While they always end up looking pretty good, I can never get them to look perfect in all email clients, and it takes a long time to do all that testing and make sure everything looks good. All of those images, fonts, and colors look totally different in Gmail than they do in Lotus and totally different in Yahoo than Apple Mail for iPad. And really, within each of those email programs, there are a ton of settings that will change the way an email looks. Some people block images and some people block all HTML markup.
The best way to get around this is to really just stick to the basics. Include some elements to brand the email and make it clear from whom the email is coming; use images sparingly, and not to convey crucial information, and leave the rest to simple text.
Seriously, Keep It Simple
Along with keeping your design and code simple, keep the messaging simple. Asking a supporter to donate and sign a petition and call her senator and come to a rally is a little overwhelming; she's more likely to do nothing.
Instead, keep your asks simple: say what you want, explain briefly why they should give it to you, and give them plenty of opportunity to take action. Stick to one, maybe two, asks per email to avoid overwhelming people.
Make Sure Your Email Gets to People
What's the point in spending all that time in effort making a nice email template that works in every browser and email client and crafting the perfect asks if your email gets caught in someone's spam filter? Make sure that doesn't happen to you by taking basic steps like including a text-only version of your email, making the "From" information is a real person, and so on. It's easy to skip some of these steps and think that it doesn't matter, but once you get blocked for spam on someone's email address, it will be almost impossible to reach them again.
Maybe I'm Old Fashioned
A simple post-card campaign can do wonders. Post-cards are cheap and easy to print in any quantity, and there is no spam filter to foil your plan. It's a great way to get additional information and build a relationship with a supporter. Think about how many emails you get each day, and compare that to how much mail you get. I get about a gajillion emails a day, but only a few pieces of mail. Feel free to send me some pretty postcards.