1. Clear
When writing or speaking to someone, be clear about your goal or message. What is your purpose in communicating with this person? If you're not sure, then your audience won't be sure either.
To be clear, try to minimize the number of ideas in each sentence. Make sure that it's easy for your reader to understand your meaning. People shouldn't have to "read between the lines" and make assumptions on their own to understand what you're trying to say.
Bad Example
Hi John,
I wanted to write you a quick note about Daniel, who's working in your department. He's a great asset, and I'd like to talk to you more about him when you have time.
Best,
Skip
What is this email about? Well, we're not sure. First, if there are multiple Daniels in John's department, John won't know who Skip is talking about.