I was just going to post this same question the other day but I see now there is no definitive answer. Here's what I can do to maximize the shave in that area:

I stretch the skin of the neck towards the opposite side. For instance, if I need to shave the right side, I have a blade in the left hand and use the right hand to hold the skin on the left side of the neck and pull it to the left. This, in effect, raises stubble that's growing sideways and allows me to use upstroke to shave it.

I haven't found anything that works better. I'm not sure it's possible to do a complete BBS with my neck. The closest I get is when I use buffing with a DE and doing it in all directions. That causes overshaving so I'd rather be without BBS than doing it too often. In fact, I've done it for the last couple of days and now even using a straight today I ended up in pain. I need to stay away from DE, which I did for several months.