6. Don't use ordinary pine for exterior trim
If exposed to the weather, pine will rot whether you paint it or not. You'd think I would have known this. I do now.
Use cedar, which if properly maintained will last at least 117 years and probably more, although 117 is all I can speak to from personal experience. Other decay-resistant materials are also worth considering, including dare I say it? plastic.
Which leads me to my next point.