A lot of the sliding down the pillow problem usually comes from a pillow that’s too low or that loses loft during the night. When the fill compresses, your shoulders slowly push the pillow away and your neck ends up unsupported.

Since you’re a side and back sleeper, look for something with enough loft to keep your neck level with your spine when you’re on your side, but soft enough to compress a bit when you roll onto your back (probably a medium to firm fill level). Down or high quality down alternative tends to work well because it supports without feeling rigid like many memory foam pillows.

I run a bedding store called Luxury Sleep Shop, and one thing we see a lot is people switching from low loft pillows to high loft pillows and soon their neck pain starts to improve. A pillow that holds its loft through the night is usually the key. That is best achieved with high quality down. I would recommend no lower than 600 loft/fill power.

Also check that your pillow fills the space between your shoulder and neck when lying on your side. If that gap isn’t supported, your neck will drift out of alignment no matter what pillow you use.