A lot of guys find that when they tighten their girls' heads down for the first time, it will be facing sideways or backwards like The Exorcist. Most doll sellers will recommend Teflon tape which can help but also needs frequent reapplicaction if you have to pack away the ladies most of the time like myself. This also leads to the creaky or knocking noise in the neck if it's loosened to make her look forward.
I found a 7/8 grommet to fit perfect around the post and it sits exactly on top of the female end in the doll's neck. The important part is making sure it sits on top of the metal part and doesn't squish the top of the neck down. Since it's flexible, you can tighten it considerably before it stops. This allows you to slightly oertighten and then back up to where you want her face. It almost never moves, unless we end up trying a lot of positions and I'm, uh, in a hurry at the time... It should be mentioned that I also use Teflon tape, this isn't to replace it, just make it better and not require a half-roll of it.
The only drawback I've found is a very slight discoloration on the TPE where it does make contact with the rubber. I ordered stain remover and will try it on the spot. Even if I can't remove it, I think the tiny stain is worth the convenience this method offer.