Another tip I learned from a dress maker who worked for several fashion designers was to lay your finger against the left edge of your foot when coming to the end of the seam. The seam will not veer at all. It's a habit now and I forget I'm doing it. Another tip she gave me was always hold the top piece of a long seam up off the bottom fabric, the two fabric only going together under the foot. The two fabrics is cut the same size will always sew exactly the same length.