Originally Posted by mgburg

1 post-level (two bubble-levels, 90° separate from each other on an "L-frame") 3 small stakes, 120° part and about 1/2 the height of the post from the the base, then enough twine/string/wire to form 3 loops, one loop per stake and back to the post.

Set the height of the 3 loops of twine/string/wire about 1/2 way up the post, then using a stick in each loop (about 6" from the high side), twist the corresponding "loop" to the point that the post is vertical/plumb, then "capture" the stick in its final position to keep it from unwraveling...

Once the concrete has set, remove the loops and stakes and move on to the next post...let the concrete set 24 hours, then start attaching the cross-pieces.