What if any additional things can I do to insure the perfect "American Arborvitae"?

Question

I bought some about 6" tall. Planted, watered and thought I was taking good care of them. They turned brown and died.

Answer 1

...should be grown in areas with considerable atmospheric moisture as well as soil moisture; requires deep well-drained soil; thrives in marshy loam; full sun... ---they probably suffered from transplant shock. when planting, soil fill-in should be tamped down very hard and tight to ensure good root contact. then soak, then add additional soil to eliminate air pockets, add a palmful of fertilizer, form a tree ring, and mulch