Make sure your code doesn’t contain any ActiveX. Double-check form submit buttons, pop-up window links, image pre-loaders, etc. and make sure they’re clean.
Avoid Flash. Embedded Flash doesn’t work consistently in most email clients. Your newsletter doesn’t need Flash, to be relevant and have a great impact.
Use absolute, not relative, links. Relative links open the risk of all links and images being broken in an email because the mail client won’t know where to find them.