I recently had my hairline waxed so that the Lace Wig Straps would stick to my skin without slipping. The straps are made with a very thin polyurethane that does not adhere well once there's a oil build-up. I recommend wiping the straps with alcohol periodically to prevent the slippage.
Waxing the neck helps to keep the natural oils from penetrating on the neck area where the hair grows. This keeps the straps in place without sliding up and causing the wig to slip backward.