The most radical approach is also the most reliable. If you are offline while your read your email, your email client might try to fetch the revealing images, but without success. And if no images are requested from the server, there is no log of you reading the message.
Unfortunately, this approach is rather inconvenient and not always feasible (in a corporate environment for example, or at school).
Use a Non-HTML-Capable Email Client
Just as radical and probably carrying even more inconvenience is it to say good-bye to your HTML-enabled email client.
If your email client can only display text, it won't even get the idea of requesting an image from some remote server (what's an image?).
Today's best email clients all support HTML, though. But you can still protect your privacy.
Configure Your Email Client for Privacy
Even if you do not want to go offline every time your read mail and do not want to switch to Pine, there are some things you can do and settings you can tweak to configure your email client of choice for maximum privacy:
(Updated December 2011)