Here is what I do:
Cut top strip (sashing) 1"
Cut back strip 1 5/8"
Fold & press back strip.
Align the raw edges of the backing strip with the edge of your block, strip goes in back.
Now align the 1" strip on top of the block, right sides together. From the top you will have: 1" strip face down, block face up and the folded strip.
Sew 1/4" seam. Use a walking foot if you have one.
Now sew the second block to the other long edge of your 1" strip, right sides together.
Sew 1/4" seam. Use a walking foot if you have one, take care not to catch the folded strip in your seam.
Lay the 2 blocks face down. If you cut your 1" strip correctly and sewed your 1/4" seams, the 2 raw edges of your block will butt together.
Now press the folded strip over to cover the seam. If you cut this strip 1 5/8" it will be just wide enough to cover the seams and have just a bit over. I press this well,(You can use washable glue to hold it if you like.) then Stitch in the Ditch from the right side to secure it.
You wind up with a 1/2" sashing strip on the front.
This is how I quilted these quilts, there's a close up of the back of one of the quilts. You really can't see the backing strip if you use the same fabric.
http://www.quiltingboard.com/t-103513-1.htm