I have been quilting for more than 30 years and still have that problem from time to time. It is very frustrating. Being sure that all my strips are cut accurately helps. So I straighten the grain of my fabrics before I cut the strips. The other important things is being sure when you make your blocks that they too are cut correctly so the pieces don't get wonky or stretchy as you sew them together. These too things will almost grantee successful finishes. Also being sure to square up your blocks so they are all the same is a big help to this problem.