Writing and talking are two very different things; it's bad advice to tell people to write as they talk. What makes talking and writing so very different are, at least, that:
1) Writing is a monologue; talking is a conversation. It's extremely rare one would talk in the form of monologue, except when giving a speech (and even in that case, the audience can usually ask questions).
2) When talking with someone, you have an immense array of non-verbal communication tools at your disposal (body language, eye contact, hands movements, speed of talking, pauses, etc. etc. etc.); in writing you have none of those, so everything you want to say, you have to write down.
1) Writing is a monologue; talking is a conversation. It's extremely rare one would talk in the form of monologue, except when giving a speech (and even in that case, the audience can usually ask questions).
2) When talking with someone, you have an immense array of non-verbal communication tools at your disposal (body language, eye contact, hands movements, speed of talking, pauses, etc. etc. etc.); in writing you have none of those, so everything you want to say, you have to write down.
Of course one should use simple, usual words, and (maybe) short sentences. But that has to be the oldest "tip" there is, in any language; there's even a .gov website dedicated to it: http://www.plainlanguage.gov/howto/guidelines/FederalPLGuide...