Take advantage of your program's formatting tools. Experiment. But keep it simple.
- Don't type in all caps if possible; it's considered yelling.
- Avoid dark backgrounds with light type (yes, just like on web sites)
- Use an easy-to-read font. Stay away from scripts unless you are using it for one or two words.
- I keep emails from people that I think are effective so that I can learn from them.
- Keep it short too - if you want a reply within a reasonable amount of time. (you don't leave long voice mail messages, do you?)