Here are Orwell's rules, edited:
1. Never use a metaphor, simile, or other figure of speech which you are used to seeing in print. You don't need cliches.
2. Never use a long word where a short one will do. Avoid long words.
3. If it is possible to cut a word out, always cut it out.
4. Never use the passive where you can use the active. Write in the now.
5. Never use a foreign phrase, a scientific word or a jargon word if you can think of an everyday English equivalent. When in doubt, say it clearly.
6. Break any of these rules sooner than say anything outright barbarous. Better to be interesting than to follow these rules.
The reason business writing is horrible is that people are afraid.
Afraid to say what they mean, because they might be criticized for it.
Afraid to be misunderstood, to be accused of saying what they didn't mean, because they might be criticized for it.
Orwell was on the right track. Just say it. Say it clearly. Say it now. Say it without fear of being criticized and say it without being boring.
If the goal is no feedback, then say nothing. Don't write the memo.
If the goal is to communicate, then say what you mean.
My best tip is this: buy a cheap digital recorder. Say what you want to say, as if the person you seek to persuade is standing there, listening. Then type that up. Simplify. Send.