For me, it would be:
1. Always assume that the person that you're talking to has NO CLUE what you're talking about, unless you specifically know otherwise.
Or, in other words, don't tell a rocket scientist how to build rockets, but assume that your boss won't understand what you're saying if you talk about it.
(Fun fact: Don't assume that people know what you're saying! Did you know that the Chinese don't have 2 words for "mustache" and "beard"? They use the same word, and get VERY confused when you mention that someone has one, but not the other. They also probably won't understand the difference until you explain it to them visually.)
2. Never write instructions. Write them, make pictures/visuals out of them, record yourself reading them, etc.
People don't learn just 1 way (and they generally don't read everything you write anyway), so present the information in as many ways as possible to make sure that you reach them... at least until you figure out what their particular learning style is. ^_~ lol
3. Too much information is much better than not enough information.
It's a lot easier to let someone pick and choose what is important for them to know on a project, than it is to have them find that their information is lacking, and because of that, they did something wrong and wasted time. It will annoy them a bit at first, but they will get used to it, and it will be FAR less annoying than finding out that their hard work has been wasted. ^_~*
So what about you? What are your top 3 communication tips? ^_^