This post used to be twice as long. Why? Because I follow my own rules. Most documents, letters, brochures, emails, are far longer, more involved, more convoluted and more detailed than necessary. Just like that sentence.
But, seriously, when writing for an external audience you have to remember that's what they are - EXTERNAL! You cannot assume that because they work with you as clients or customers that they are privy to all of your internal vocabulary and slang.
As you write that communication piece that will be heading out of the office, remember that even though you think you have used plain language, most likely you have still used industry lingo, internal vocabulary and terms - even if they seem plain and simple to you. Get someone external to edit it - a professional editor or someone in the 'general' public who knows nothing about the subject matter. I often ask my fiance (a millwright) to review my client communications pieces. If he has to ask what a term means - the piece isn't simple enough for external people.
Now that you have eliminated all of the technical, specific and internal language - make it shorter. Chances are, when you eliminated an industry-specific word, you substituted an explanatory sentence, rather than another single word. Go back and edit now to make it clear and concise. Take out useless words like very, frankly, actually, honestly, truthfully, really, quite, so, very, somewhat, seems, utterly, practically, basically, and rather.
Also watch your phrases. There is no need to say 'end result' when results only happen at the end. There is also no need to say 'added bonus' because that is what bonus means.
Becoming a savvy, powerful writer will not happen overnight, but if you edit carefully, ask for help and write actively, not passively - your readers will thank you.
