1. Don't include a subject line - Why give them a heads up about the content? Better yet, this is a great way to help SPAM filters send your mail straight to the Junk Folder. Also makes it harder for them to recall the mail for any archive searches down the road.
2. Don't have a Signature Line with contact information automatically attached - Making them take extra time to look you up in their Contacts list or Google you is a great way to cause a bit of mild irritation.
3. Avoid using correct grammar, spell check, or punctuation - This is a TXT world after all. show how hip & busy u r by avoiding capitalization, speil-cheecking, punctuation and other useful ways of making your email easier to read. Also great when sending out job feelers to potential employers.
4. Add loads of frills and colors - colorful backgrounds with low contrast text is always a hit! Be sure to use crazy fonts and make it really small. Especially useful for folks reading their email from mobile devices.
5. Pass on those "neat" forwarded messages about cute kitties or world ending viruses - Be sure to not clear out the 435 lines of previous receivers of the message, too. And never check out http://snopes.com to check the story's veracity before passing it on to friends and clients.

Those of you not in this situation take heed, you could gain some client good will by quickly counter-acting these email faux pas of your competitors.