First, you've got to sound like you're chatting with your reader, like you're giving them an unfiltered stream-of-consciousness access to your ideas as you think them. Second, on no account should you actually do that.

Eliezer is one of the masters at this; his essays are littered with phrases like "y'know" and "pretty much", but they're way too tight to be hastily published first drafts (or maybe I'm wrong and Eliezer is one of the few people in the world who can do this; chances are you're not). You've got to put a lot of work into making something look that spontaneous.

This is also important to keep in mind when writing fictional dialogue - the reader has to perceive the conversation as authentic, forgetting that people don't actually tend to speak in a manner that would be at all interesting to read. Basically, you have to borrow the tone of a real conversation by keeping only about 5-10% of the interjections and filler, and using them only when it helps keep a statement believable.