I cast on one extra stitch and then slip the first cast on to the right needle and pass the extra cast on stitch over it. It totally closes the join.
Sometimes, if I am using a seed or garter stitch border I will knit a few rows straight, join and sew up the teeny "seam". This works well if you have a hard time keeping the first row from twisting too.
Mom to the late Cyrus the Fluffweiler CGC forever in my heart and James the Alpha cat