The chin can be one of the last areas to conquer.

What I do is take the skin around my cheek at the jawline and stretch it backwards. Then I take the razor and hold it in a vertical position with the tip point slightly toward the center of the chin (so it might look like the razor is in the 1 o'clock/7 o'clock position) and shave from left to center. I do the same thing on the right side of my face and shave from right to center.

As for the area at the corners of the mouth. I don't stretch those usually. I just lay the razor flat with the spine on my skin and take very light and short strokes. If I have to stretch it, I puff up those areas instead of stretching (use extreme caution in doing this though).

Another way to stretch the chin is to place fingers of one hand on both sides of the chin, say right at the corners of the mouth, and gently stretch back toward the ears on either side of the face. Be careful of the hand position relative to the razor on this one.