ORIGINAL: Connery
As others have said it takes more heat to shrink Monokote than another product like Ultracoat. To do any covering job properly and to be able to get out all of those wrinkles, a heat gun is a must. For the best covered finish (with any product) is to seal the edges with the iron, and then shrink the rest of it until its taught with the heat gun. I see too many people trying to shrink an entire wing or fuselage with just an iron.
I agree on the heat gun thing. I also used a heat gun from the hardware store for a while. It had two settings: way too hot or not hot enough. I finally got an actual Hobbico heat gun. The heat guns sold for shrinking covering are the right temperature and work much better, at least for me anyhow.