Like the poster above me stated, it sounds like you're letting your fire get too close to your target temperature before regulating air flow. By doing this, you're squelching the oxygen flow to your fire, thus producing a lot of white smoke.
I used to smoke on a Kamado style grill. Yes, it's different from a WSM, but the minion method worked like a champ for me. I know a lot of other WSM owners who swear by that method, too. Simply put, you start a small fire in a different pit or chimney starter, move that small fire to the center of a huge coal and wood bed in your pit, then let the small fire slowly go to work, igniting the huge coal bed a little bit at a time. For more details, google "minion method."