I think the main thing is to try to stretch your lip up over the lower teeth leaving just the area to be shaved exposed. Maybe easier said than done. For the areas to the sides, I try to twist my jaw to the left and right with the lip still curled up over the lower teeth. I find that as I get older and heavier, this area of my face becomes more problematic and I have to be more creative with my stretching.

Good luck.