How do you get nice sharp corners when turning two sewn pieces of fabric together?

Well, the obvious answer is to stitch right to the corner and stop with the needle down in the fabric, pick the presser foot up and pivot the fabrics 90 degrees then continue stitching. When the seam is finished, clip the corner off and turn the fabrics right side out. BUT, THIS WILL NOT GIVE YOU A NICE SHARP CORNER!

The correct way to do it is to stop stitching with the needle down in the fabric just before reaching the corner, pick the presser foot up and pivot the fabrics 45 degrees and take 2 stitches that are 1 mm long (yes, you have to adjust the stitch length). Stop with the needle down in the fabric and pivot the fabrics another 45 degrees. Be sure to readjust your stitch length and continue stitching. When the seam is finished, clip the corner off and turn the fabric right side out.

The "rule" to remember that stitching a sharp corner will result in a blunt corner, but stitching a blunt corner will turn perfectly square.

Practice this on scraps by using a thin leaded pencil to mark your stitching lines so you'll know exactly where you'll need to stop for the corners.