First off, I'm right handed. And going against the grain in the aread between my neck and my right jaw is always a pain. I've been shaving with a straight for almost three months, and I still seem to cut myself there every now and then while the rest of my face is just fine.
So, what I do is take the razor and bend it backwards on itself to make a 'V' shape. Then I hold it upside down. Imagine you're making the peace sign with you fingers and you hold it facing down with the knuckle of your index finger against your face.
And then I go against the grain using a scything motion. I get a smooth shave on my neck that I can't seem to get any way else. And it's funny, but I feel like I have more control holding the blade upside down on this pass.