The Mason Technologist

#1. Add a hotlinked text-line at the top of every message (can be in tiny type) that says something such as "Did your email system mangle this newsletter? See it Online."
Note: Don't say "Click Here" because that wording as a hotlink in an HTML-heavy message can get you filtered as a spammer in some systems.

#2. Remind your opt-ins incessantly to add your send email address to their address book which means your email won't be filtered or hurt in most systems. Best places -- on "thank you" landing page they see when they opt-in, and in your welcome-to-our-list autoresponder. Some mailers such as TJMaxx.com also add a reminder in text at the top of every HTML email they send. The bad news is, most email recipients are lazy especially when it comes to commercial email. They leave address settings on default, they don't bother clicking to see images. Fewer than 5% of your recipients will heed you if you do both #1 and #2. But heck, those are very motivated prospects and may be among your most valuable list members.
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#3. Get yourself and your email creative team sample email accounts at Gmail, AOL 9.0 etc. Keep all settings on default so you see worst possible scenario of what may happen to your HTML creative. Often you'll find yourself adding more text wording to messages than you had initially. Images simply can't be relied on for the entire message. Neither can HTML click buttons. If a recipient can't see the button, they're not going to click.