I personally never really found a topical product that would help with this.
For me, it's important to make sure that you only go over those spots once (the back of the neck). I also read somewhere that if you DON'T pull that skin tight when shaving, you'll be less likely to have irritation (the rationale being that pulling the skin tight will cut the hair closer and increase the risk of ingrown hairs and irritation).
I've read that using a razor with fewer blades will lead to less irritation, although for me, I've found that the quattro and the fusion give me the best shave... Maybe the fewer blades thing is true for some people, but not me.
I notice that the top and sides of my head get "stubblier" more quickly than the lower neck area. I don't shave everyday because I get irritated on the back of my neck if I do, but I find that I can "clean things up" with an electric shaver on the in between days to keep the back and sides from being to stubbly. This might allow you to go longer between razor shaves but still stay pretty smooth most of the time.