I use a digital kitchen scale for weighing similar to: http://www.bedbathandbeyond.com/product.asp?SKU=40596600&RN=1151&
The challenge is to get the bird to still a few seconds. I bring the bird into my mudroom and have the scales sitting on the floor. It sometimes takes a few times putting the bird on the scales. My scales are just large enough for me to get both of the bird's feet on it.
Don Schrider uses a square bucket & puts the bird in the bucket and uses hanging scales.
I use a large cardboard box, set the box on the scale, subtract the box weight. I am just using an old baby scale I bought at a swap meet for $1. I tested it, it is accurate to about 1/8 pound. Good enough!