Okay, that helps us drop the formal bureaucratic language and write in plain English, but it's not enough.
We need to think of appropriate informality.
In workshops, I like to create an imaginary line. One side of the room represents stuffy or lofty (unnecessarily academic) language. The other side represents casual language like, 'Gidday mate'. I advocate writing in a band around the middle.
Why a band?
Because we need some flexibility.
Let's imagine you are writing a business case for your executive management team and it's going to be a big decision for them.
For the executive team at their meeting you will need more formality than in a routine email to a colleague, or a client you chat with every few days. For the executives' meeting, you shouldn't contract so many words and you'll need slightly more formal words. Your language may be more like a presentation than conversation, but not stuffy or lofty.
That slightly more formal language is a way of respecting the formality of their meeting and the seriousness of the topic. Compare it with getting dressed up for a wedding.
Let's modify the conventional advice.
'Write it as you would say it to those readers - in the context in which they will read it.'
So, if you would be embarrassed to say it to those readers at their meeting (or over a coffee) don't write it.
Here's a small complication if you are writing for an international audience.
Business writing in Asian and some European cultures tends to be much more formal than in New Zealand or Australia. Meet them part way or the informality of your language will be a distraction.
You can see the formality or informality in the sign-offs. The French expect a sign-off for business letters as formal as, 'Please accept the expression of my distinguished sentiments'. It doesn't translate well. You might treat it as the equivalent of 'Yours faithfully'. In sign-offs, go with the flow.