Well, aside from the standard "make sure your grammar is all in check", after you've written a few paragraphs of conversation, look to see how your sentences start. Do you repeat the same openings? "I don't know why you persist in..." then "I think you should..." then "I was wondering..." and so on. If you can find ways to vary the wording, it makes for more interesting reading, as it would also make for more interesting conversation. That's one tip. Would you like another?