I've found with my neck I have to pull the skin very tight to get any sort of effective shave, otherwise it comes out stubbly and indifferent. I usually shave down, then up, then maybe across, or maybe up or down again.

As I mentioned once before, I also tend to have to press a bit harder when I shave my neck as compared to the rest of my face, maybe because the hairs are tougher, or maybe just because the skin is pulled so tight. This is even with a freshly honed razor.

You'll get it in time, though, it just takes practice. I've been doing this for a few months now, and under the lips and my moustache still give me problems.