If you use a DE use a DE twice a week. I have the same problem as you do but I use a straight. Every week I shave it and it lasts a very long time with a straight. The back of your neck has pretty thick skinned, smooth and is tough. It might sound hard to do using a straight but it is quite simple and I have never cut myself either which is more than I can say about my face.
Take Care,
Richard
I've thought, just from some trial runs to see how I'm going to have to position myself, that a straight razor would be much more practical for this. The grip makes more sense. But, alas, I'm not a straight shaver. DE only. Is the learning curve worth picking up straights primarily for neck shaving?