Are your blocks all the same size and square before you are attaching the sashing? If not, you need to do this. Sashing won't line up if the blocks aren't straight to begin with.
Next, are you cutting the sashing to length first? Let's say your blocks are 8 1/2 inches. You need to cut your sashing to 8 1/2 inches. You can't just take a big long strip, sew it to your block and cut it off. You run into the same issues that you would if you did this with the borders.
You can try attaching the sashing and keystones to the blocks, rather than dealing with that big long sashing + keystone strip. To do this, you would attach a sashing strip to the right side of the block. Next take a sashing strip with a cornerstone attached and sew it to the top. Basically you are making a bigger block. Then sew these together.
And finally are you cutting little 1 1/2" squares??? I find this easier. Lets use that same pretend 8 1/2" block size. Cut an 8 1/2 inch strip of sashing fabric, then a 1 1/2" strip of cornerstone fabric. sew the 2 together, then slice this joined strip into 1 1/2" pieces. This is a good place to use spray starch before you do the final slice.
sorry this is so long.....