First off, think about what it is you want to achieve with your memo or email. What’s the point? Who exactly is your audience? What do you want the reader to do as a result?
Organize your thoughts BEFORE you start writing. Jot down a few notes or a short outline. This will make the writing go a lot quicker for you.
Write the same way that you speak. The best writing most closely resembles normal, everyday speech. If it’s too formal, it becomes harder to understand. But watch out - that doesn’t mean to throw in a bunch of slang like “ain’t”, “gonna”, etc. That’s a bit too loose.
Be brief and concise. Make your point, support it and move on. This is not a time for a bunch of fluff (there’s never a time for a bunch of fluff). You’re not impressing anyone with it.
Write and re-write. It’s really not often that we get it absolutely right the first time. Write your memo, re-read it, let it sit, and re-read it again. Especially if it’s a critical piece. Don’t send something out when there’s a possibility that it might come back to haunt you. Get it right the first time.
BE POSITIVE, even if you’re conveying bad news. No one likes to read negative memos, so it will sound much worse than it actually is to the recipient. Negative writing often back fires on the writer by reflecting poorly on him and in return, loosing the message for the recipient.