Cornerstones help a LOT. You can even do cornerstones the same color as the sashing.
Another way is don't attach the whole row of blocks & side sashing then a long strip of bottom sash. Apply sashing (with or without cornerstones) to the side, then add the bottom sashing to the individual block. Now join the blocks with 2 sides sashed into rows & join the rows matching the seams.
In all cases, don't just sew a strip of fabric to your block & whack off the excess. Measure and cut your sashing strips before you apply them.