"It seems to be hard for most people to write in spoken language. So perhaps the best solution is to write your first draft the way you usually would, then afterward look at each sentence and ask "Is this the way I'd say this if I were talking to a friend?" ..."

The trick that works for me is setting the font-color to be the same as the background color, blak on black for example. Then you can write without being able to go back and correct what you've written. With the result, that you will write almost the way as you speak. Requires touch typing though.

I even made a colorscheme for Vim for this and using it when writing in my diary or personal emails. Does wonders.