Indeed, "write like you talk" is not quite the right way to go about it. In my experience that usually leads to a stream-of-consciousness mess.
However, one thing I have found almost universally helps my students is similar: read what you have written OUT LOUD. Is it halting and hard to read? Confusing and obtuse? Then you should probably fix it. There seems to be some magic about actually speaking the words out loud (as opposed to just reading it like normal editing) that helps a lot of people figure out when a piece of writing is bad.
However, one thing I have found almost universally helps my students is similar: read what you have written OUT LOUD. Is it halting and hard to read? Confusing and obtuse? Then you should probably fix it. There seems to be some magic about actually speaking the words out loud (as opposed to just reading it like normal editing) that helps a lot of people figure out when a piece of writing is bad.